Badass Decay: The Deep Devils in Frontiers are powerful and brutal units fitting for a Brutal Bonus Level. The Deep Devils in Vengeance that are summoned by the Deep Devil's Reef are far weaker, the Greenfins and Redspines being rather squishy and dealing a lot less damage, and the Bluegale only being able to summon a single stormcloud instead of covering a huge area in them. Justified, since they had suffered a huge loss after Leviathan's demise and are still slowly recovering from their defeat.
After 15 brutal waves in the Sunken Citadel, Leviathan comes out like this. His extremely slow movement, lack of useful tricks (his only ability - stopping to disable certain towers next to the coast for a second - makes him stop while doing so) and weak minions (Greenfins, Deviltides, Redspines, the Demonic Spiders only appear if you take too long) make him pale in comparison to his own Elite Mooks.
Vasile the Vampire Lord from the Shadowmoon campaign counts, for much of the same reason. He's not even as large as most of the other bosses and has no really useful tricks up his sleeve. Don't get too confident, you WILL be slaughtered when he comes out, just by his entourage of twenty or so Vampiresa.
Disappointing Last Level: The cave stages in Frontiers. Much of the puzzle-like elements to beat a level are gone. Your towers and powers have leveled up beyond anything the enemy can throw at you and the levels can be swept through sheer firepower because there are no ultra-heavy armored foes and only one caster type, which downplays the Tactical RockPaperScissors aspect of the previous stages a lot. The initial funds are much higher, which allows for much less strict and more relaxed planning in the early waves.
Ensemble Dark Horse: Big Bad Vez'nan is quite a popular character, due to being a hammy villain who spouts pop culture references. It got to the point where he became playable in Origins, and is the Villain Protagonist of Vengeance.
Even Better Sequel: Many reviews have considered Frontiers to be better than the already-awesome original.
The Vampiresa enemies have Absolute Cleavage and a much bigger chest size than the good female vampire Lucrezia. Margosa from Vengeance is more modestly dressed than the Vampiresa but still less so than Lucrezia.
If you look closely, the Scourgers in Origins are actually quite Stripperiffic.
In contrast, most of the good-aligned female enemies in Vengeance are more modestly dressed, such as the Ice Witch, the Valkyrie, and the Devoted Priest, with the only Stripperiffic enemy being the Northern Huntress (and even then they show no cleavage unlike the Vampiresa).
Fan-Preferred Couple: Eridan/Alleria and Reg'Son/Lynn are the two most popular pairings in the community, despite the lack of any canon evidence (not that it matters, since past the first game, the heroes barely have any profile). Even thedevs support them.
There's a pretty good reason why Mactans never fights you directly unlike Sarelgaz in the first game. She's only as big as a Son of Sarelgaz. While those are durable, armored, and magically resistant, a single one isn't much trouble to defeat for a late-game defense. Thus, Mactans avoids direct combat with your towers as they would end her very quickly.
The levels in which you obtain new towers in Vengeance make quite a good bit of sense.
You obtain the heavily-armored Dark Knights right after Vez'nan successfully takes over the Dwarves' Clockwork Factory and its metal-smelting facilities. Presumably, the metalworking facilities are used to make the heavy armor.
Beating the final level of the Dwarven Kingdom gets you the Melting Furnace, presumably also constructed from said metal — and it so happens that the next world is the frigid, frozen North where a furnace would be quite helpful in combating the cold.
The Specters' Mausoleum is obtained after beating Frozen Rapids, where Vez'nan loses a good bit of his army thanks to the battle breaking the ice floes. So of course, he puts those souls to good use.
Bone Flingers are obtained after Dragons' Boneyard is beaten. There's quite a lot of bones in that map, and Vez'nan decided to make good use of them.
Elite Harassers are unlocked at the start of Silveroak Outpost. As mortal enemies of the Silveroak Elves and especially Alleria Swiftwind (who you fight in the level), they're probably out for revenge since their loss in Origins and joined Vez'nan's army for that.
Orc Shamans are unlocked after City of Lozagon is beaten. As a magic-using city full of mages and other enchanted stuff, the dark army probably found some elemental mastery stuff for their Orcs to read and learn magic from.
The Cavalier enemies have a surprisingly low amount of health compared to the Paladin that dismounts when killed. This is because you're attacking the horse, and horses are actually quite fragile animals thanks to being unable to heal a broken leg easily.
Despite being a Bluegale, who are immune to magic, Jun'Pai lacks any magic resistance. His bio states that the Trident he wields would kill anyone not capable of handling the intense magic within and that he has to endure the energy running through it. As such, he loses all his magic resistance by handling his trident in order to prevent it from killing him.
Vez'nan. He periodically casts a spell that renders a large number of your towers temporarily unusable (which is very annoying) and kills normal soldiers in one hit, but he doesn't have that much health.
Ulguk'Hai also falls into this. He has a shield that absorbs all hits on him. He only puts it down when he's fighting your troops.
Baj'Nimen the Hateful from Origins. He regularly spams an AOE skill similar to Rain of Fire on your hero and group of units, but what's worse is that he's one of two bosses in the series that can reliably heal himself for a good amount from time to time, prolonging the fight.
Gerald Lightseeker in Vengeance. Not difficult and lacks a strong AoE, but has a good bit of physical armor, and like Baj'Nimen above he can also reliably heal himself from time to time to prolong the fight. You can also spam the bandit mercenaries on him to make things less of a hassle.
The Winter Queen. She has very low HP for a boss (4000, the lowest in the series), but she freezes your towers at an extremely fast rate, prolonging the fight.
Good Bad Bugs: Vengeance has one that occurs if both the Specters Mausoleum's Possession and the Wicked Sisters' Froggification hit an enemy at the same time. It'll cause the affected target to permanently switch sides and fight for you until they die or exit the area, instead of wearing off normally. This also happens should a possessed Frozen Heart die, its soul (and subsequently regenerated Frozen Heart) will permanently fight for you.
Hilarious in Hindsight: A fan-made game idea for the series had an enemy called the Hyporus, an icy monster with arms made of ice that turned troops it killed into icy monsters. The Frozen Nightmare mini-campaign for Vengeance introduces the Ice Reaper, an icy spirit with icy arms that turned troops it killed into icy Apex Shards.
Perhaps the most severe example of this would be the Saurian Deathcoils, whose attack is a powerful, armor-piercing crossbow bolt that has a range of nearly half the screen. As such, they can easily pick off your barracks, skeletons, and heroes before you even have a chance to move them. For this, they're considered to be one of the most annoying and hated enemies in the entire series, though they thankfully only appear in one level.
Artilleries don't fire where the enemy is located (or into the biggest density of units), they fire at where the enemy will be. This means that should the enemy stop or be intercepted by your units or perform a burst of speed, you will miss that shot and waste the splash damage. A big part of why Druid towers in Origins are rather useless is that none of the level 4 upgrades have any gimmicks to work around this.
The Bloodsydian Crystals in the Hulking Rage campaign. When they activate, they can either: 1. Upgrade gnoll burners and reavers around them into Blood Gnolls. 2. Kill all gnoll burners and reavers around them. 3. Heal a group of enemies. 4. Teleport a huge group of enemies back. 5. Teleport a huge group of enemies forwards. The first effect can overwhelm you with a bunch of Blood Gnolls, while the last effect is especially nasty in a Heroic or Iron challenge and it teleports a group of Hyenas, which will then charge past your troops at the back and cause a defeat.
The second half of Vengeance's final level has King Denas' mortar troops dropping flaming Tar and Feathers on your towers to disable them. In order to free your tower, you either need to wait 40 seconds or pay 125 gold. Unlike Umbra's and the Spider Goddess's tower-destroying, this happens far more frequently. Better hope they didn't hit one of key importance, and if they did, better hope you didn't spend too much of it upgrading your stuff! Even worse, it lasts indefinitely until cleared in the Heroic and Iron Challenges, but it's thankfully reduced to 30 gold in the Iron Challenge.
Snow Golems in the Frozen Nightmare expansion initially appear on several spots on the map including areas near your exit. They're dormant until a certain wave passes, after which they get up as a slow but powerful enemy with very high HP that drops a paltry 25 gold on defeat. To get rid of them prematurely you can pay a huge 200 gold to drop a bomb on their dormant bodies, which gives you a Sadistic Choice: Spend a huge amount of gold or fight an annoyingly tough enemy that drops chump change which can be spawned near your exit. Thankfully, the cost needed to bomb them is reduced to 100 gold in Heroic challenges, and they do not appear in Iron Challenges at all.
Sequel Difficulty Drop: Compared to the original, Frontiers and Origins, Vengeance has enemies that generally aren't as tough (Giant Mooks have less HP overall although they sport armor) and allows you to use specialized towers from the get-go instead of having to level a tower to its maximum level to get its specialization.
That One Achievement: In Frontiers, there are a few achievements that stand out among the players as the hardest to get due to them requiring a lot of luck and/or time:
"Ali Baba" requires your Assassins to pickpocket up to a whooping 10000 gold in total. At max upgrade, the upgrade only has a 40% chance to trigger while yielding only 1-3 gold per success. You can accelerate the process by building Assassin Guilds on nearly every available spot, but it is a very risky move as doing so will cripple your defenses and possibly end your run early if not careful.
"Let it bleed!" needs you to kill 100 enemies with bleed damage. Emphasis on the word kill, meaning that you risk having your other towers land the killing blow if in range or even by normal attacks. This is only possible with Templars having the Arterial Strike upgrade and the hero Cronan who has a whip attack. You can do the same as stated above but you have to take note of the risks of doing such an action.
"Highlander" requires one of your Templars to resurrect 5 times in a row with the Holy Grail upgrade. However, it only has a 40% chance of triggering upon death. Pray that luck is on your side.
"Grim Reaper" requires your Death Knights to accumulate 99 kills. Same issue as with "Let it bleed!" above. Not to mention, this is an upgrade for the Necromancer towers, which are Mage towers that can steal kills.
That One Attack: In Vengeance, we have the Anurian Chasers' leap attack. When they get within a rather generous distance to your troops, they perform a leap that not only clears a good bit of the lane, but the landing also deals a moderately damaging area Shockwave Stomp. The main reason why it's an attack you don't want to see is that they appear in huge numbers, and all of them using this can and will whittle down Barracks troops while easily sending the group of frogs forwards past a third to half of your towers. Worst of all, this attack has little to no cooldown and they can immediately hop to your next set of troops to bypass more area and plow through them.
Lord Blackburn from the original is up there as one of the toughest bosses in the series. He has a very dangerous ability to temporarily stun your units and up to EIGHT towers within a massive range, deals heavy area damage making him hard to block even with Paladins, gets a rather short distance to your exit, has 90% physical resistance (due to being a Tin Tyrant) which makes magic towers a must, and several of his flunkies are highly resistant to magic. Speaking of which, a number of his flunkies are annoying (Black Hags) or outright dangerous (Lycans, Fallen Knights), compounding things further. Mercifully, he's the only boss in the first game who has all his minions die and the waves of enemies stop appearing once he goes down.
Umbra, the Final Boss of Frontiers. It seems like a standard Flunky Boss at first, but when you "kill" it, it explodes into many small blobs which you must destroy as much of because they'll reform into the boss (but its Maximum Health will be lower if you killed some of them). The battle only gets harder as it goes on as it then uses Eye Beams to outright destroy your towers (This is one of two enemies in the entire series that can do so), making it harder to kill his smaller Mooks. In order to beat him, you need to keep killing him and his blobs before he destroys your defenses and his hordes get through.
Maginicia Shores from Vengeance is often seen as a ridiculous jump in difficulty from the main game, even by Elite Stage standards. You are just simply bum-rushed by a horde of heavily armored enemies, but they each have their own tricks so you can't just pound them with magic (Chasers leap onto your troops to bypass your towers and Wardens have their shields replaced by Infusers). The waves involving Wardens and Infusers can get especially hairy, as the game chucks so many of them at once despite having very good armor.
The Druid towers in Origins are considered rather useless due to having nothing to circumvent the Artificial Stupidity of the Artillery towers not to mention being unable to hit air units at allnote In the original, Big Bertha has homing missiles while Tesla has the Chain Lightning. In Frontiers, DWAARP has a 360-degree splash damage while Battle mech has the aforementioned homing missiles. In Vengeance, the Goblin Zeppelin can be positioned to affect troops effectively, the Melting Furnace uses a 360-degree splash, the Rotten Forest has a similar 360-degree area of effect In Origins both the level 4 druids uses the same inaccurate rock throw with only clobber as the reliable splash damage with the Burst Arrows being considered to be far more reliable splash damage than the Druids. This unfortunately has the side effect of making swarms that much more difficult compared to other games but particularly the spiders who are both fast and easily shrug off the splash damage from the Burst Arrows.
In a similar vein, the Goblin Rocket Riders in Vengeance are seen as one of the more useless towers. Like the Druid towers in Origins, they share similar weaknesses but are even less reliable. Nitro Boosters are a nerfed version of the Dragonbreath or Wasp missiles, having a much shorter range and the inability to target air units. Minefield is unreliable due to random placement on the track and can outright miss enemies that don't go close to them. Finally, unlike Origins' Druid towers that at least had abilities to stall or stun (Runed Bears and Clobber), Rocket Riders don't even have a way to do either. The other artillery towers in the Melting Furnace, Rotten Forest, and the Goblin War Zeppelin are seen to be more useful thanks to having wider, more reliable Area of Effects and stuns, or dealing more damage while being more versatile.
The Templars in Frontiers are pretty universally agreed to be outclassed by the Assassins for most situations. Assassins have similar survivability (due to their dodge rate), way better damage, and their upgrades cost a lot less. Additionally, the Assassins go into stealth when not moving or in combat, so they can't be targeted by ranged attacks. The Templars' only real saving grace is using them against enemies with area-of-effect attacks, which the Assassins can't dodge.