These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The second game Nerfs them by quite a bit, as well as giving you a Fear Spell which can push back Stealthed Ninjas... Until the Ninjas got buffed back again with extra health and longer invisibility time. Still, a Fear spell is very effective at ruining their day.
To a lesser extent, Assassins. They have a 50% chance to evade any attack. If the Random Number God has a bad day, this is very nasty. note Fortunately, there is the Frenzy spell, which makes your towers spam projectiles and can effectively counter that ability. And they cannot evade Standard Status Effects, either. And just as fortunately, their evasion ability was changed to reduce damage by a variable percentage instead of completely evading it.
Iron Guards in the sequel. These Champion robots are basically slower Ninjas (Stealths on first hit) with a load of health and they can fly over water, taking a short cut to the exit! They can't be inflicted with fear either, so no pushing them backwards. Which means the only way to get rid of these in the final boss fight is via Meteor Spam, as the gems are very close to the entrance for flying foes.
Cursed Treasure 2's Wizards after the update. These guys now have a lot of health, and their special ability gives them a speed buff... which they can release in a burst, boosting the speed of AND healing all nearby allies. When a huge number of these appear, they'll pretty much heal and speed buff each other unless you can damage them faster than they can heal.
Amazons, when they appear in numbers. As soon as they lose at least half their health, their armor gets knocked off. When this happens they charm all towers in a radius around them, making those towers unable to attack for 8 precious seconds. Sure, you can use a Fear spell to get rid of the charm, but this costs your precious Mana. If you don't have any mana to spare, you're helpless as these bitches run past your charmed towers! They were nerfednote Charm lasts for 5 seconds now, and a tower hit with fear becomes charm immune for 2 seconds in the update, thankfully.
Divers, in Stage 20, due to their shortcut being very near your gems. These guys traverse water by going under, becoming invisible to towersnote Splash Damage still hits! They also have quite a lot of health to boot. In stages 19 and 21, they're Goddamned Bats.
Designated Villain: The only evil thing the player is shown as doing is killing the people who are trying to steal his/her gems.
Goddamned Bats: Cursed Treasure 2 brings us flying enemies like the Valkyrie and the Kite Druid. These can fly over water, taking a shortcut to your gems!
Healer/buffer enemies tend to use their heal/buff skills very often, compared to the previous game.
In the sequel, Rogues can be this in the early waves. Not only are they fast, but they now have a Sprint ability that temporarily makes them even faster.
Game Breaker: The Chilling Crypt & third charge combo. The Chilling Crypt has a chance of scaring the enemies for a short time, which increases greatly when they get a third charge.
Heck, the entire Undead tree in general. It reduces cost of Frenzy and and increases Frenzy Speed, helps with mana regen (Use in conjunction with Orc's Cut Out Study and Cut Out Mastery for fast gold and clearing of areas), increases chances to fear, reduces Crypt costs and finally, increases Fear chances as well as giving a Freeze chance!
The Undead Skill tree is no slouch in the sequel either. It now trades Frenzy for Fear (which can be upgraded to take away 1/5 of an enemy's Maximum health when it affects a foe), allows Crypts a small chance of instantly killing an enemy (works on Champions but not bosses), and has the usual cost reduction and freeze chance abilitynote Combine the Freeze Chance and the Instant Kill chance with the very fast firing rate of the Lightning Crypt to destroy champions. For larger mobs, see below. Furthermore, it also gets a skill which makes undead orbs circle around your gem cave- useful for finishing off weakened foes who somehow make it through. Not to mention the maxed out terror tower's shot can rebound infinitely as long as there's new foes near the last bounce, and each bounce can apply the usual effects, which can utterly destroy large mobs as they clump together due to being terrorized.
Funny Moments: Truly there is no greater joy than having multiple fear crypts then watching those little bastards run backwards screaming like ninnies. Repeatedly.
Most Annoying Sound: There hardly is one non-annoying sound effect - most are cheesy or noise and some are a bit gruesome.
The douchebag laugh when they pick up a gem, the mocking laugh when they get away with it.
The "kiss" sound that Amazons make when you break their armor. This sound means that your towers are probably charmed and will refuse to attack for 8 seconds (or hit with fear), which is a very bad thing.
That One Level: Stage 20, "The Two Towers◊" (before the redesign), if you're going for a Brilliant. It got so bad that they redid the level a bit and made it easier.
Mana is very hard to come by thanks to a lack of mana pools. The redesign gave the player one to make it less frustrating.
Not enough spaces for the very powerful Crypts (read: only two)
2 Lighthouses that give a speed boost to 3 foes within a large range (See yellow circles), each requiring 7 uses of Cut Out . Taking over these will cause them to damage up to foes in the radius.
2 pirate ships that spawn enemies, each requiring 10 uses of Cut Out.
Divers (spawned by Pirate ships) are invisible while submerged, and the route to your gems has loads of water, giving very few chances for your towers to attack before they touch the gems. (See the red routes)
The Divers tend to appear with Cooks. Cooks will heal the divers, making it harder to kill them before they reach your gems. You'll be forced to use the Fear Spell many times, which reduces the Mana you need to Cut Out the lighthouses and ships!
You'll probably be forced to use a Fear Spell against the Smuggler Bosses, and maybe even a Meteor Spell against the Iron Guards (who get a very nice shortcut to your gems).