YMMV / Orcs Must Die!

• Awesome Music: The battle themes will get you pumped to make orcs die, especially the first battle theme, called "Castlemania".
• Catharsis Factor: Giant tightly packed army of orcs, meet Fireball. Or Chain Lightning. Or an entire hallway of traps. Or a giant pendulum mace. Or smashing ceiling. Or acid pits. Or...
• Demonic Spiders:
  • Gnoll Hunters. Hunters are stupidly fast, hit hard to make matters worse they slow anything they touch which makes running away virtually impossible once they hit you and they're tougher than the regular orcs too. Hunters also actively target Guardians and they can kill Archers and Dwarves in mere two to three hits. Even Paladins are quickly downed by these things.
  • Gnoll Grenadiers will harass you from a distance with much less powerful bombs, but the damage will add up very fast and there's the fact the bombs can break Barricades if you ignore them. Grenadiers will happily kill ranks of Archers and Dwarves as well if left alone.
  • If you're going for a five-skull ranking (which requires that you let nothing through the rift), Kobolds can quickly become this. They hopelessly outrun most traps and Paladins. You'd better hope the archers stop them if you're not around to do it yourself, or spend a ton of cash laying down Grinders and Brimstone (and then keep an eye out anyway to make sure the little buggers don't overwhelm them with sheer numbers).
  • Kobold Sappers. Say hello to an enemy that is fast like a Kobold Runner, hits for a third of your health, comes in large numbers and that will happily nuke the shit our of your Barricades and/or Guardians!
  • In the sequel, post Family Ties Booster Pack, Hobgoblin Healers become this whenever they are paired in a wave with a large damage sponge creature, such as an ogre (or worse.) Desperately burning down its huge reserve of Hit Points, getting close to finishing it off, only to have it restored back to full health by a couple of Hobgoblin Healers you never noticed approaching from behind who are hanging out on the safe end of your Death Course...
  • Cyclops Mages are a tremendous headache since they attack from a very long range with homing projectiles which not only hurts a lot but also locks your mana regeneration for a few precious seconds. Unlike Gnolls they don't get any special sound to announce their entrance in the map, but you'll learn to be on your toes whenever you fight two of these things.
• Game-Breaker:
  • Multiple upgraded Swinging Maces with Tar Pits on the floor. Fill every possible chokepoint with the combination of these two traps, and the only enemies you will ever need to kill personally are the occasional ogres who manage to get through. You might well go through entire levels where you literally never have to fire a shot (besides for the first waves, during which you're still setting up your corridor of death).
  • Spore traps automatically convert any enemy who steps on them to your side — even Brawn Hilda, an enemy type that typically is immune to mind control. If you last long enough to coat a large enough area with these traps, you can literally convert an entire wave of orcs to fight for you, which can render Endless mode trivial.
  • The third game gives us an upgrade to the Swinging Maces + Tar Pits combo with the advent of the Laser Ceiling. Not only do the lasers cover a wider area with the proper upgrade, but they spew continuous magical death, making it that much harder for things to slip by. At baseline, Kobolds will die en masse to them, especially those Sappers; with damage upgrades, they will chew through even ogres in a matter of seconds. Set up some archers to pelt them as they struggle through the Laser Hallway, and absolutely nothing will get through.
• Goddamned Bats:
  • Frost Ogres. While Fire Ogres can be tackled relatively quickly with the Ice Amulet, Frost Ogres are bulky enough to soak lots of Fire Bracer's spells (enough to deplete your mana gauge) and basically forces you to either be very careful with them or bring something else to deal with them.
  • The Fire Babies. They are slow, do little damage, and die in one hit, but they fly, they are small and hard to target, and they come in big numbers. They are not much a threat on their own, but their flight means they have to be dealt with sooner rather than later, distracting the player from other threats they might be focusing on. Fortunately, the Lightning Ring takes them down efficiently. They are easier to deal with in the sequel thanks to the Dwarven Missile Launcher weapon, which provides the player with an anti-air flak secondary fire.
  • The sequel adds bats of different elemental types, all immune to their own element. They almost always force the player to pick Archer Guardians or something specific to counter them as they can very well fly over your traps or bombard you unexpectedly, making a bad situation worse.
  • Earth Lords in the sequel are a hassle to deal with since once they die, they split into two Earth Elementals which then splits into two Earthlings each. Later waves in Endless mode often include as many as 4 Earth Lords in one wave, and depending on your trap and weapon load-out you always have to let out at least one of them.
  • Almost literal bats in Three, few traps can accurately target flying enemies. Thankfully they only turn up in the Post Game and DLC.
• Good Bad Bugs: Gnoll Grenadiers can't tell which floor their target is on, which can lead to them throwing bombs into thin air one floor below your archers/what have you. Perfect if you have other dangerous enemies to take care of who won't be a threat to your guardians.