The cannons, modelled after the Dwarf bombard 500mm Big Bertha from Kingdom Rush sit outside Denas's castle entrance. In the Campaign, the only appear in the second part of the battle (beginning of wave 9), once Vez'nan has destroyed the outer city walls. In Iron Challenge and Heroic Challenge they appear from the start.
The cannons will launch barrels at random towers, covering them in a burning mixture of tar and feathers. While they are covered, the tower cannot be interacted with, nor will it attack or use any of its skills. There are a few exceptions to this, however. A rally point in use on a tower will negate the inability to attack, as it's only the tower base that is affected by the tar, but they cannot be repositioned.
In the Campaign, a tarred and feathered tower will free itself after 40 seconds. In the Heroic and Iron Challenges, a tarred and feathered tower will stay that way until cleared.
- The units spawned by Orc Warriors Dens, Dark Knights, Elite Harassers and the Grim Cemetery will still fight, but will not respawn if killed.
- The Wicked Sisters will attack as normal, and as the flying sister is not a unit, she cannot be killed. This also applies to the Goblin War Zeppelin balloon.
- The Rotten Forest damage area will continue to have an effect.
Purchasing a soapy bucket costs 125 gold in the Campaign and Heroic Challenge, but only 30 gold in the Iron Challenge. In the Campaign, a tower will remain covered for 40 seconds, meaning that gold can be saved if a less important tower is hit. In the Heroic and Iron Challenges, a tower will remain covered until it is cleaned, so it is recommended to uncover the strongest towers first.