It has been a staple of Real-Time Strategy games ever since the seminal Dune II that the player must construct a base to provide units right on the field during the battle. These bases most often resemble small cities devoted to military production rather than any kind of realistic field base. Only rarely are these in place at the beginning of a given scenario. The enemy usually does not suffer from this restriction, but will thankfully refrain from attacking until you can establish your own base.
Highly unrealistic and often not even addressed, this is still necessary to some fans. The feeling of building and managing a city in wartime is oft preferable as a fighting experience than going through with tactics and strategy. A great many varieties of Applied Phlebotinum are invoked as justification, generally some way of ultra-fast manufacturing or teleporting assets onto the field. This is such a staple of Real-Time Strategy that games lacking it are sometimes categorized as being in a different genre altogether - specifically, Real Time Tactics.
Several of the best-regarded games in the genre are ones that do something interesting with the concept. In Battlezone, the struggle for the Applied Phlebotinum behind such wonders leads to a plot where the Cold War is secretly duked out in hovertanks across the solar system. In Total Annihilation, the ability to build armies out of nowhere is not an incongruity but the basis of the gameplay mechanics. If one constructor can build another constructor, then those two can build four, those eight, those sixteen...
Common fantastic/unrealistic elements include the following:
The initial building being created from a large, slow-moving vehicle. A staple of Dune II and its descendants. Creating another one usually has huge requirements to prevent early expansions.
Arbitrary restrictions on placement of buildings, usually called the control radius or somesuch. Trope CodifierDune II could justify it by restricting your construction to rock, instead of building your houses on sand. Most games don't have such justifications. Increasing this radius is part of why StarCraft tells you to construct those additional pylons.
Having to build entire power plants right on the field, instead of hooking up to the power grid or relying on field generators.
Having to gather resources during the battle to fund the war effort. Particularly ridiculous in Red Alert and its sequel, with ore and gems growing out of the ground.
Having to build some arbitrary "support" building before being allowed to field a particular unit.
The Trope Namer is Starcraft, which tells you incessantly that "You must construct additional pylons" in order to build more Protoss units. Pylons not only raise your Arbitrary Headcount Limit but provide building limitations by supplying power to other structures; structures cannot be built outside a Pylon's power radius and will go offline if the Pylon(s) powering it are destroyed. The game justifies Zerg building limitations by requiring them to build on Creep, a carpet of purple Meat Moss. Their supply cap is "control", provided by the Overlord air units. Terrans rely on Supply Depots to extend the unit cap (and act as ad-hoc walls), but can plonk their buildings down pretty much anywhere there's room—heck, some of them can lift off and fly somewhere else!
Also Warcraft with farms (Alliance), burrows (Horde), moonwells (Night Elves) and ziggurats (Scourge).
The weird thing is that, canonically, you aren't constructing Pylons. You're warping in already-constructed pylons from somewhere else. This is made weirder in the expansion packs and sequels, where—again, canonically—the place you're warping them in from has been overrun by hostile forces. This unrealism is taken to its most logical extent in Starcraft II, where in one Bad Future mission you fighting the Milky Way's Last Stand. Literally every remaining member of the Protoss race lives in your base... except for the ones you are magically warping in from somewhere else.
For the 4X-based Rise of Nations, you build whole cities and infrastructure instead of normal bases: the cities expand your territory, your infrastructure increases your resource revenue, the resource increase only applies to farms, mines and lumber fields built within a certain radius of your city, you can only build within your own land. However, building a state is really the point; the game is really aiming at "RTS-styleCivilization" than Command & Conquer-style war. The Easy Logistics of battle are averted as your units suffer attrition damage when inside enemy turf, which is nullified if you keep a Supply Wagon nearby. The fact that nothing enters your military production buildings is still kinda strange though (helicopters never land, for instance).
In Total Annihilation all sides start out with a Commander. The Commander builds factories that build construction units that build more factories and power plants and defenses. While there is no arbitrary limit on the size of your base, you are restricted to building mobile units from factories only. Unlike many other RTS games, resource collection is mostly preformed by stationary buildings - your construction units can reclaim wreckage of destroyed units, rocks, miscellaneous metallic structures, trees and flora, and the bodies of dead alien creatures (the serpents and scorpions) for a set amount of metal or energy, but otherwise you need to depend on stationary buildings for a steady stream of resources. Resource management is an important strategy, as the player who can control more of the metal deposits can get the upper hand.
The Age of Empires series of course does this. While this is more forgivable when it's meant to show a whole war or at least a section of the war it makes less sense when playing scenarios which are meant to be just a battle, which you can then win by building a wonder and have it stand for 100 years!
It also has a literal "Construct additional pylons" message in "You need to build more houses!" One solution to this annoyance: play the Huns, who are nomads and don't require houses.
Lampshaded in the Command & Conquer: Red Alert 3 tutorial, in which the Soviet Tank wonders why the most powerful military forces must gather ore in the middle of a war and wondering what's in that stuff anyways. He is immediately shot by the other tanks for "asking stupid questions".
Halo Wars has fixed bases, each of which contains plots for 4-6 buildings. No construction units are present. Instead, the base gets flown in from your Cool Ship (or teleported in the case of the bad guys.)
War Wind requires the player to set up living quarters for every couple of units, which they then hire from elsewhere or recruit from people in the overworld.
Plants vs. Zombies has Sunflowers (and for night stages, Sun-Shrooms), which do nothing but produce Sun, which is required to buy plants for attacking or blocking. It's not unusual to have more than a third of the field completely covered in Sunflowers on more advanced stages.
Played completely straight in the upcoming real-time strategy game Achron.
Parodied in Sluggy Freelance when Torg, stranded in medieval England and pretending to be the Warlord of Mercia, has to lead his army into battle.
Torg: Alright, we'll need some of the townsfolk to chop down trees, mine for gold, and set up solar collectors in case we need to build more troops. Do we have any dragons yet?
In the second The Lord Of The Rings: The Battle For Middle-Earth game, the amount of farms (or mallorn trees in the case of the elves, or mines in the case of the dwarves) determines how many command points you have, capping at 1000.
Dawn of War also has most of these gameplay elements. It also does away with traditional resource gathering (mostly — you still build field generators). The resource you must gather is controlled territory (represented by Strategic Points). The more of the map you hold, the quicker your requisition points come in.
It should be noted that several buildings and units (like infantry) are not constructed, but shot from orbit. Which is kind of typical of Warhammer 40,000.
Averted in DOW 2 and its expansions; the main campaigns feature no base building at all, simply capturing strategic points and at most setting up an automated defense turret. Multiplayer only featured your main production building and whatever structure you could build on captured points.
In LEGO Rock Raiders, you are only allowed to teleport in 9 Rock Raiders before you have to construct a Support Station. After that, you get ten additional worker spaces for each Support Station constructed.
Aztec Wars simplifies the usual system. On each map you are limited to a number of pre-placed bases, which can only produce the weakest infantry unit, but can be expanded into one of two or three types: Village, City and Fortress. Each of the types has a different selection of buildings and units available. Getting money is achieved simply by putting down the Farm or Mine buildings, which then produce cash automatically (though they can only be built on a specific type of terrain, and give more income when placed on especially fertile spots, so terrain in the game is sort-of the equivalent of limited map resources).
Spellforce plays this and ArbitraryHeadcountLimit straight, but it's justified/explained quite nicely: You're a magical rune warrior, and you can summon your forces out of (magical) thin air. You only need buildings to create weapons and armor for your troops.
Sudden Strike has no base building at all; you have a set number of forces to use in a given scenario and must make do with those. Enemy bases and factories can be captured though, repaired and in some scenarios put to work.
There are also chokepoints (spot on the map or airfield) through which reinforcements can be sent from outside the map. This allows for realistic cutting of the supply lines by attacking the said units before they can hold their ground, or simply dropping a lot of land mines at that spot.
The earlier Myth: The Fallen Lords uses a similar model, with the exception that you're occasionally given reinforcements at scripted points, and can pick your forces using a point system in multiplayer.
Ground Control has the player select their units at the beginning of each level, and then fly them in from orbital Drop Ships. Buildings and units are never constructed.
Its sequel, however, does throw in some unit construction. In this case, the only structures are pre-built on the map, some of which can be garrisoned, or portable defense and sensor devices, moved about by transports. Victory points can be held with ground units; holding them adds to the requisition point income. Said points are used to purchase units, which are loaded on the off-map Drop Ship, or to buy upgrades for the ship itself. The Drop Ship hauls in the requested units and drops them at the designated LZs, which become de-facto bases. The resource management comes from the space on the LZ, the "income tax" taken when the number of units on the field grows, and the time it takes for the drop ship to make round trips; some upgrades can expand the cargo bay or increase the speed.
World in Conflict completely eschews base building, but does involve building small field fortifications at control points. While it is not realistic to have everything flown onto the field of battle during the fighting, it is a good deal better than pumping them out on the fly from factories.
Sacrifice only has three buildings: Altars, Manaliths, and Shrines, all of which are placed only on top of Mana Fountains. Manaliths allow players to tap, well, mana. Altars and Shrines may be used to "convert" enemy souls, which are required for the players' wizard to summon creatures and are the only resource in the game besides mana. Also, wizards are killed by sacrificing a unit on their altar. Hence the name of the game.
And all those things you build, you build with magic.
The Nintendo Wars series has all the buildings on a map in place prior to deployment, which you'll have to capture with infantry to use. Although factories produce military hardware seemingly out of money instantly, you can't build more of the factories themselves.
They even explained a quirk of the Nintendo Wars series this way, in particular why the player simply can't "carry over" units built from a factory in the previous battle; weapons and vehicles built by these factories function only within range of factory they were built and can't be engineered otherwise (by the rag-tag protagonists at any rate). Taking the units out of range renders them, as one character puts it, "a useless lump of metal.". In the case of infantry, their guns don't work outside the area.
In the Total War series there's no building units once you get to the battlefield. The only forces you get are the ones you've built and brought to the battle on the grand campaign map.
There are no bases at all in the Mech Commander games. There is no in-game unit construction. The best you can do is in the second game, where you can disable an enemy 'Mech and have one of your pilots hijack it. There is in-game repairing of units, but even that is limited. It is purely a tactical combat game.
It is possible to capture certain buildings, at least in the first game - turret control buildings and repair gantries are particularly useful. Gate controls...not so much, unless you have jump jets.
In Battle Realms you built the buildings to train soldiers, however, in order to actually get soldiers, you have to tell the peasants to train in the building. Apparently, the unit cost was the food/water that recruit needed.
Played weirdly straight with the peasants, though. A hut is effectively a peasant factory. Although it produces peasants faster when it already contains some, it still produces them when it's empty. And of course constructing buildings requires wood, although in an aversion of Easy Logistics the peasants have to carry the wood to the construction site. Fortunately, they're smart enough to do that on their own.
Mostly averted/justified in Sierra's Outpost 2, a SimCity/Warcraft-esque RTS IN SPACE! with humanity colonizing another planet to survive. The planet they live on closely resembles Mars and thus their buildings have to be connected via airtight passageways, and humans mostly only work in buildings as overseers. All construction/resource gathering/fighting work is done by robots. As such, your base is a small city, and while there isn't necessarily a hard cap on population, not satisfying the colonists' various needs, such as housing and medical care, as well as paying attention to the military element, can lead to morale and workforce problems that can destroy a colony from within.
Averted in Darwinia and Multiwinia, which have almost no construction element at all. Most buildings are pre-existing and need only be taken control of, bar a very few deploy-ables such as the turret weapons.
End War has no buildings (except Uplinks, which are mission objectives, and cover, but they're not stuff you build), and only the Arbitrary Headcount Limit, your Command Points, and your available reinforcements deciding how many units you can deploy to a given battlefield.
Empire at War, at least in Story and Galactic Conquest modes, because everything is built/trained/recruited between battles, on the Galactic map. Space Skirmish is the only exception, since you have to constantly upgrade your space station during battle, build mining facilities on asteroids, and gain control of and build on defense satellites. Even then, you at least start off with (a level one) station (and optionally some free fighters) and credits to build stuff with with.
Subverted by Original War, an RTS from 2001. You build a base in there, but nothing's really going to do itself: you want to store supplies? You need a warehouse, and a couple of guys who will bring the stuff inside. You want to make tanks? Then you need a factory, and a guy who can build vehicles. Oh, and once you have it, you need a driver, too. And no building is gonna build itself, you know. Oh, have I mentioned you need power plants? And fuel for those? And, if you want to build a ton of infantrymen and rush the enemy base with them... Too bad, you can't, the best you can do is to recruit some local apemen. But you need someone to tame them, and to find them, too.
Supreme Commander, spiritual successor to the aforementioned Total Annihilation, has as a central plot-point the ACU (Armored Command Units), a marvel of technology that can create enormous armies out of nowhere (actually, exploiting the current landscapes of its mass and energy). This theoretically would reduce the casualties of war to, well, one single commander per battle. The problem is that the Aeon are fond of "Purging" non-believers (and thus, killing civilians in civilian structures) while the UEF is not above targeting civilian structures for the moral effect. Even the Cybran, in their bandit form, not under the fatherly leadership of Brain-in-a-futuristic-holographic-Jar Dr. Gustaf Brackman, tend to target civilians. You, ultimately, don't need the civilian structures to raise your headcount as your army is completely automated. They are there only for story-driven and aesthetic purposes.
The Applied Phlebotinum is that the Commanders are basically the largest thing that can be teleported safely via Quantum Gates. So it stands to reason that an ACU arrives with only the most basic to start building (that is: an small energy reactor, mass generator, and simple engineer tool). With those, it can construct more-complex factories that can themselves construct more-complex and specialized units. The ACU itself can upgrade its simple Engineer tool (among other things, like chest-mounted-deathray) once it has acquired sufficient resources from the immediate vicinities.
The Settlers games largely avert this trope by having the goal be to build an effective settlement with an efficient economy. In fact, in the first Settlers, it was often possible to win a level without significant military engagement because the computer character would eventually simply run their own economy into the ground.
The whole X-COM series. You need to build facilities to house your soldiers, scientists, engineers, craft and various supplies, you do not build people - these are recruited and delivered using some normal transport - and building your own craft in the midgame takes relatively realistic amount of time (they are pretty much fighters or transport fighters ranging from 14k - 34k man hours per one - this only counts the work your engineers do though. The construction costs you materials and money, which means there are probably lots of subcontractors who supply you with various non-alien parts in the design). The third installment, Apocalypse, expands on this. You can now no longer recruit anonymous guys from a practically bottom-less pool. You have applications which you browse to find the best skilled people and if there are no applications, you can't recruit anyone, period. Until the pool replenishes that is. The various vehicles have an headcount limit, but far from arbitrary - you can only stick so many guys in the cramped transport. In Apocalypse, this could be remedied by landing several transports at once or ferrying the agents group after group. In the tactical portion, you were limited to IIRC 6 squads of 6 people each though...
Homeworld avoids actual base building by wrapping most everything up into the Mothership, which contains everything necessary for building any of the other ships in the game as well as eliminating the need for "houses". Very helpful since everything takes place in the 3-dimensional vacuum of space. The only two other ships that stay with the Mothership to form the "base" are the Research Ships, which are needed to advance up the technology tree, and the Sensors Array, which floats nearby and boosts... well your sensors. Carriers often form part of the home base as well, but they usually are needed at the front to fuel smaller ships and strengthen the assault directly.
Cataclysm and HW 2 push back into traditional base building territory by having you construct modules on their respective Motherships before any thing else can be done. This internalized the research and sensor ships making the Mothership a true all in one field base construct. The two games also let the Mothership move in single player, though Carriers still typically take the role of building on the front line.
It's said that the Support Modules in Cataclysm didn't represent housing for crew, but rather the command and control capability needed to coordinate more units.
Part of the problem is the ambiguity as to whether the ships are controlled remotely from the mothership or directly crewed. Incidental radio chatter and the terrified screams when the Beast infects would indicate direct crews, but other things like the missions involving control disruption, the drifting exploding ships after the motherships are destroyed, and the inability to capture certain ships due to incompatible systems indicate remote control.
The game's background implies that Cataclysm's two Somtaaw motherships were a home away from home for a significant proportion of Kiith Somtaaw, who found themselves driven into the depths of space thanks to their limited clout in the New Diamid. Significant numbers were carried aboard each mothership in cryostasis and rotated in and out in half-year-long shifts.
Panzer General averts this, where units are requisitioned from HQ and have to be paid in "prestige" (i.e. the more successful you are, the more willing your superiors are to give you more units. This would work even better if one of the Soviet tanks wasn'tfree.
Blitzkrieg, a WWII RTS that Needs More Love, lacked bases or resources of any kind. You have all your units at the start, and if you lose them they're gone. (Except for infantry, who can be resupplied as long as one member of the squad is still alive.) Sometimes you would get extra units in the form of 'reinforcements' arriving, but that was it.
In Z you can't construct factories. You can rebuild a destroyed bridge and construct defensive guns, though. Building units is justified as it takes pretty long and they're mostly robots. You can also put a robot inside an abandoned vehicle or gun.
Shattered Galaxy, marketed as the first MMORTS, made all units persistent and thus obsoleted base-building.
In Hamurabi the player has to eventually buy additional farmland to feed his growing population.