Considering the huge number of heroes and towers in the series, there are bound to be a few that break the game.
- Ashbite the dragon seems to be one in terms of heroes. He does a lot of area damage with his attacks (one of which can instakill), can slow groups of enemies by a large amount and flies, making 80% of enemies unable to attack him at all. Of course, you need to buy him for a lot of money (even more than the game itself!) to use him. On the other hand, he attacks very slowly and is quite vulnerable to ranged attacks...
- The expansion brings us Bonehart the Dracolich, who is basically the crowd-control counterpart to the single-target DPS Ashbite. He can summon bone minions to block foes, takes Ranged attacks better than Ashbite, and all his attacks deal Splash Damage and give enemies a disease that makes them explode for more damage on death (which can cause a chain reaction). However, he costs just as much as Ashbite.
- Dante may be another contender. Although he's squishy, he's able to give all allies around him a 50% damage boost which works even when he's deadnote , deal the most DPS of a hero with his rapid fire and Silver Bullet (which instakills werewolves and Lycans), and can also disable enemy abilities and completely shut down their resistances. Unfortunately, all his abilities can only effect one target at a time.
- Alric can also be a good contender due to his amazing durability, second highest DPS in the game, the ability to reflect melee damage, and the ability to stall up to 4 enemies thanks to his Sand Warriors. Speaking of his Sand Warriors, they can travel down a lane (more chance to stall enemies), won't give birth to Reapers when killed by Parasytes (the only other unit to share this ability is the skeletons), and has a cooldown time of 10 seconds, making it very spammable (though they give out after 8 seconds). The absolute deal breaker is that he's the first hero you get. However, like all melee mighty glaciers, he has slow movement speed and can still be picked off by archers.
- Phoenix in Origins can also be considered as one. For starters, her attacks deal true damage, which means enemies won't be able to resist the damage, and they burn. She also has access to attacks that can reach across the map. Her hero spell also has the fastest cooldown time at 20 seconds, meaning you can spam it as you will. Fire attacks won't burn her. She also has the fastest respawn time of all the heroes across the KR series at 5 seconds. When she dies, she dives down to the ground and explodes, effectively damaging enemies. Afterwards, she turns into a flaming egg that still damages enemies. Finally, when respawning, said egg explodes, yet again damaging enemies.
- Vez'nan in Origins isn't exceptional by himself, but his hero spell summons an incredibly powerful demon with both excellent crowd control and single target DPS, who can take out almost anything short of a boss by himself.
- Pre-nerf Asra from Vengeance used to be the best free hero in the game and better than several of the paid heroes. With her passive Shadow Dance, she could teleport anywhere on the map to clear out enemies. Her damage was decent, her Spider Bite was a poison effect that could never be removed and was essentially a One-Hit Kill, Shield of Shadows allowed her to tank damage for a brief period to get Spider Bite in, Quiver of Sorrow permanently removed enemy armor at 6 points per shot at maximum level, and her Ultimate Spell Toxic Rain did heavy armor-piercing poison damage on an area and had a ridiculously short cooldown of 28 seconds with Ultimate Power upgrade. The developers knew of this, and nerfed her by halving the amount of armor that Quiver of Sorrow removed while greatly increasing Toxic Rain's cooldown.
- Jigou from Vengeance. If you want a character who can stall hordes and tank Giant Mooks and bosses, he's your yeti. With 750 HP and 24% armor at max level and his Yeti Blood passive that regenerates up to an insane 30 HP per second even in combat (not counting the usual regeneration out of combat), he's exceptionally tough — enough to indefinitely tank a MechaDwarf MK.9, for starters. Frozen Breath and Earth Slam stun groups and the latter also deals great damage. Then there's Glacial Form which activates at <30% HP. It makes him completely invincible for 9 seconds, during which he can and will regenerate allowing him to tank even bosses for over 15+ seconds. Finally, his Ice Zone Ultimate deals hefty damage to an area and greatly decreases speed of enemies in it for 5 seconds. His weakness is that he's the slowest character in the game and fares poorly against Airborne Mooks.
- Beresad in Vengeance is sort of like Ashbite, having similar abilities to him. What makes him different from and more useful than Ashbite is his Hero Spell "Hellfire Shockwave". When activated, it'll cause a rain of fire for a short moment, and every enemy on the screen will take a great bit of burning damage that bypasses armor and magic resistance. Oftentimes, this will heavily soften up or outright kill the weaker mooks, allowing your towers and yourself more breathing room to focus on the Giant Mooks.
- Eiskalt in Vengeance. Like Beresad, he can fly, but what makes him tremendously useful is his abilities. His Fierce Breath passive ability gives his basic attacks a huge AoE splash and said attacks are very powerful for a basic attack. Cold Fury helps to crowd control enemies by slowing them, Ice Peaks deals 30% of an enemy's maximum HP as armor-piercing damage which makes it handy against Giant Mooks (and if you're lucky it can hit twice), and his Hero Spell Glacial Storm freezes all enemies on the screen for 12 seconds at maximum, and it has a fast recharge. Finally, all his attacks and damaging skills will slow down enemies by 30% for a short while thanks to his passive, that includes the splash from his basic attacks.
- The original game has the Tesla towers where they fire Chain Lightning towards enemies. However what makes them far better than all other towers especially artilleries is that their splash damage is reliable and completely accurate due to their nature and their damage output is particularly excellent especially when you add the Overcharge ability to the mix. Heck just placing multitudes of the will easily wipe out entire squads of enemies, even those that have high physical armor such as Demon Legions.
- Of the tower variety, the Crossbow Fort from Frontiers is considered as one. It's a level 4 archer tower that not only has the ability to do massive burst damage (a minimum of 6 bolts in quick succession, each dealing 30-40 damage) every 6 seconds, but can also boost the range and critical chance of itself and nearby towers. Even enemies with high physical armor are no match for this tower.
- In Vengeance, we have Bone Flingers. They do low damage at a rapid rate targeting random mooks in range and their upgrades are very cheap compared to several other towersnote . Their drawback of random targeting becomes an unintentional boon if there's a pesky Ice Witch, Valkyrie or Devoted Priest behind their flunkies. Their final level abilities are cheap and effective — Got Milk increases their damage (very noticeable thanks to their initially weak damage), Summon Bonem conjures a huge unit akin to the Sorcerer Mage's Earth Elemental, and The Walking Dead makes it spawn a skeleton that slowly walks up the path every few seconds. This ability is extremely good since the skeleton is essentially a barracks troop that can move up the path on its own while delaying Giant Mooks and drawing fire from ranged enemies, and if you have a lot of Bone Flingers the skeletons can slowly stack. Best of all, Bone Flingers is one of the free towers.
- Shadow Archers from Vengeance. They are Simple, yet Awesome as the very first tower you use in the game, firing single shots at enemies at a rapid rate from one of the longest ranges in the game. They were relatively cheap to upgrade and did very good damage at high levels especially against unarmored enemies. Their final level abilities are similar to the Golden Longbows from Origins — a One-Hit Kill, a Damage-Increasing Debuff mark arrow, plus an added Damage Over Time crow ability. Several maps including Heroic and Iron Challenges could be easily completed just by spamming nothing but Shadow Archers since smaller enemies died quickly, and larger ones were instakilled by Blade of Demise. Even the Crow's Nest ability had some use against the Final Boss with several of them whittling his health down at a steady rate as he took double damage from Shadow Mark. Tellingly, their upgrade cost was increasednote and Crow's nest had its damage decreased in order to balance them out.
- The Specters Mausoleum's Possession ability is generally unreliable when it comes to targeting (since it can target a weak enemy that gets in range), but if it targets a dangerous one like a Frost Giant or High Sorcerer, it will give you an incredible ally for a short duration while temporarily removing a threat. There's also the neat effect of getting a victory if the final enemy remaining in the level is possessed. It goes into this territory in the Frozen Nightmare expansion with the new enemies. Possessing an Ice Reaper can give you a small army of Apex Shards to block enemies if it tears through Draugr, spawning an allied Shard on kill. A Possessed Winter Lord will inflict a permanent health-draining status on any enemy it attacks, even on the Winter Queen herself. Finally, a Good Bad Bug occurs when a possessed Frozen Heart is killed by an enemy it's fighting, which turns off the possession timer. This gives you a permanently possessed Frozen Heart/Soul that regenerates from death and durable in both forms thanks to the Heart's high HP and the Soul's incredible armor rating.
- The Blazing Gem. Even though they're excellent against big, slow enemies with high physical armor, they're expensive and difficult to manage, since not only they target the enemy furthest from the exit, but they also won't change target until either the enemy is dead or leaves its range. Their damage is also gradual, meaning they always attack initially with a low powered beam before picking up steam. However, they excel in melting down bosses in seconds. Thanks to how bosses work in Kingdom Rush games, they're mostly slow units who start at the furthest point from the exit, making them an easy target for this tower. Since bosses always come after the final wave, you're more likely to have extra money to spend. Just put down one or two of this tower right at the boss' entry lane, upgrade to level 4, buy the Gem of Amplification upgrade, and watch as the bosses fall down just a few steps ahead from their starting point. It's quite telling that in the final level of the main campaign, there are 3 tower building spots right by the boss entrance point.
- The Swamp Thing tower is a very powerful hybrid Barracks/Archer tower that has insane range and high tankiness, but it mainly shines as an archer tower. At maximum level, it boasts an insanely long range and hits for 90-100 damage per hit, although it has a slow rate of fire. As barracks, it sports 3000 HP at maximum, enough to tank a good few boss attacks. Its upgrades are also extremely powerful — Blinding Liquid gives it up to 60% chance to stun an enemy, potentially allowing a stunlock. Smash! gives all its attacks (both ranged and melee) a One-Hit Kill chance, similar to Golden Longbows' Crimson Sentence. Carnivore gives it back a huge amount of HP on kill, increasing survivability. Its main weaknesss are slow rate of fire, which isn't much considering the insane range and damage, being less efficient against blocking swarms, and that it's useless against Ghosts.