Best Level Ever: For players who like levels to create mazes Height of Confusion is this. It's a simple level with a shitload of free grids to create your own maze and 32 Waves full of enemies to test it.
Contested Sequel: From removing the interest resource gaining mechanic to the implementation of micro-transactions in community maps, let's just say some people didn't take kindly to the sequel.note Classic Open mode was added in 03/24 which features DG2 maps using the same interest mechanic of DG1: The Awakening
Seekers. Unreasonably tough and prone to spawning large numbers of a variety of other aliens in front of them. Even worse, the game's guns almost always tend to lock onto the expendable cannon fodder they're spawning, letting them make absurd progress practically untouched by your defenses.
In Center of Power, Seekers starts very far from the core housing which actually is problematic since it gives them more than enough time to summon hordes of other types of aliens ranging from Swarmers, to Decoys, to Rhinos and even Spires.
Spires. Even the most trivial Swarmer or Walker can become a problem as Spires give them reasonably strong shields.
Decoys. Reasonably though and they give aliens cloaking ability and without a Command tower (or Disruptor in the Boost tower in Defense Grid 2) the other towers will focus only on the Decoy ignoring the others until it dies, by that point aliens can be either past through you main line of defenses or running away with a core. To top everything enemies cloaked by the Decoy are practically invisible to Meteor and Cannon towers.
Air units are not terribly difficult to dispatch—a few missile turrets will ensure they don't touch your cores. but if they do get one of your cores, it's gone for good!
In the sequel Carriers, they drop a shield whenever they die and sometimes whatever comes next can become a huge headache to kill because of that, namely Crashers and Rumblers.
Suppressors in Defense Grid 2. They take a while to die and will most likely do so right amidst your towers, the problem? When they die they disable every tower in the vicinity for a while which is almost always enough to create a huge gap in your defenses.
The Landing Pods in Defense Grid 2. These things always drop right next to the core housing and lay aliens there. It cannot be destroyed but all damage dealt to the Landing Pod mitigates to the aliens it spawns, but long range towers (Cannon and Missiles) have the tendency to ignore them and keep firing on other aliens and sometimes you don't have the option to build short range towers close to them, to make things worse some of the Landing Pods barely stays there for a couple of seconds before starting spawning aliens. Landing Pods become even more dangerous in Frozen Core challenges as if an alien so as much touches the core, it won't float back to its housing.
In the first game, Meteor towers can easily dominate the battlefield, especially Grinder challenges. While expensive to upgrade, it has great range and a very good spread damage. Combine them with Temporal towers and good maze with Gun towers for mopping up the survivors and bulkies, relax and enjoy. On the other hand, speedsters and isolated enemies draw their fire from clusters (resulting in overkill or miss) so the balance is cleverly maintained in most scenarios.
Command towers if used properly get extremely close to this. On maximum level they give a 45% bonus on resources after kills and they have a very wide radius to help, they also help dealing with pesky Decoy and Lurkers to boot. Get a Command tower maxed and in good position and watch the cash, er, resources flow!
Missile towers with the temporal upgrade in the sequel; great reach, good rate of fire and constantly halting enemies, making them an exposed easy target for the rest of the towers.
Swarmers and Walkers are this especially early on some maps. Since they come in such large numbers it's easy for them to snatch a core one alien left after dying and slip by your defenses moving away from the map.
Rhinos in Defense Grid 1 are normally easy to deal with, but they aren't as fragile as Walkers and can, and most likely will, at least once, grab a stray core midway to the exit and endure your towers just enough to escape.
Racers. As their name implies they are fast and if they snatch a core midway to the exit it'll be very hard for your towers to kill them before they flee. They get into Demonic Spiders territory quite easily in later waves when they become more resilient and are a nightmare to deal with in the later waves of Adrenaline challenges.
Bulwarks becomes one without Gun/Cannon or Tesla towers as their shields will last an eternity without them. In Out of Guns challenges they basically force you to build Tesla towers.
In the sequel Bulwarks can easily be countered by building a Boost tower and giving it the disruptor upgrade to render their shields useless in a small area, but in Suspension you still don't have disruptors, so you are forced to rely on your Tesla towers.
Lurkers are especially annoying since they travel the map cloaked and towers will not shoot them unless they get very close to them. If they get into crowds without a mean to reveal them probably only area-of-effect towers will do anything to them.
Most Wonderful Sound: General Fletcher's praising words once the map is over is nothing short of satisfactory to hear.
That One Boss: Crashers are this for the unaware. What makes them so difficult is the fact they very rarely come alone and by the time they die they can be midway to the exit carrying three cores and leaving them as easy pickings for smaller, more fragile enemies. The worst part about Crashers? Later on they come in packs.
Turtles are stupidly tough for their speed (not as tough as Crashers but far faster) and the aliens they carry inside them often cause as much trouble as they do. On later levels they can bring Crashers and Rumblers.
Rumblers. Racers on steroids that are as tough as high grade Rhinos, without Temporal towers it's not uncommon for these things to just blaze through everything, pick cores and leave before the player even has a chance to built another tower.
Seekers later on becomes this. They can start summoning things like Racers and even high grade Bulwarks.
Juggernauts are just like Crashers, but with shields, although they are very slow they take forever to bring down even with Level 3 Towers.
It's quite telling how prepared you need to be when pretty much every boss enemy is listed here.
That One Level: Some levels in Defense Grid 1 have the aliens starting extremely close to the cores granting that your interest incoming will be severely affected. Naturally these levels tend to be very hard and annoying also due to the fact cores will be floating constantly and aliens will be in a tug-of-war with you until the very end of the map.
Several of the advanced challenges can get very, very bad, especially if you are aiming for Gold medal.
Green Towers Only challenges tend to be very taxing on one's patience...
Grinder and Super Grinder challenges for those going for the gold medal. You'll easily earn enough Resources to have 10 gold medals' worth of points, but you still need to finish with no cores lost; few things are more frustrating than being on wave 99 and failing to notice an alien heading out with a core in tow until it's too late.
Balanced Building forces you to build one type of tower before you can set a second of the same type. Add that to some levels that are really difficult on their own and you'll be pulling your hairs out of head in no time.
Some of Center of Power's advanced challenges can cause a few gray hairs.
Turnabout's advanced challenges (namely Super Grinder and Green Towers Only) are the point where you are forced into "juggling" the aliens in order to achieve the Gold Medal. Even then, getting that requires precise timing when juggling the aliens, without which these challenges are largely Luck Based Missions.
It's nothing that the application of enough gun/cannon towers and construction of a long enough maze can't handle. Try doing the Gun Crazy achievement on Height of Confusion and you'll see why they're just shy of being OP.
In the sequel the Super Grinder challenge of Overlook can be hair-pulling difficult due to Early Game Hell. You have only Boost, Gun, Inferno, Laser and Meteor towers for this mission and without Temporals to slow down Racers and Rumblers you might realize how difficult things can get and how easy is to see everything spiral out of control because of Suppressors exploding and jamming your towers in the worst moment possible.
Double Take in Defense Grid 2, especially on Hard and Elite. You won't have access to any special ability here, so no orbital laser, tower empowering, instant resources, slowdown beam or target locking. To make things worse there are two core housings, so you also need to place towers carefully as you won't be able to fix a mistake with a special ability.
The Woobie: Many of the places you visit had special importance to the General (where he lived, where his family lived ( and were overrun), where his mind was uploaded into a computer ( and where his body rests), the sites of battles from the previous war... It's hard to not feel bad for him when you hear his reaction to revisiting those places.
Neither Cai or Simon has it easy, their home world was overrun by the aliens and they are desperately struggling to save survivors.