The Anti-Tank Gun is a defensive building that is good against Heavy Cavalry.
The Anti-Tank Gun starts as the Ballista Tower (at level 1 and 2), then the Cannon Tower (at level 3 and 4) and finally, the Anti-Tank Gun (at level 5 - 8).
The Anti-Tank Gun has a very low rate of fire and very high damage.
The Anti-Tank Gun's high damage means that it is effective against enemy units with high hitpoints such as Heavy Cavalry, but its low attack speed means that it can be easily swarmed by large number of enemy troops such as Heavy Infantry. Therefore, putting this behind walls is a good idea.
Level 1: shows a stone tower with multiple windows without any glass in them and a blue line on top of it. There is a big wooden crossbow on top of that with an arrow in it. The back/feather part is red.
Level 2: the top and bottom of the stone tower are covered in metal. the back of the crossbow turns into a white painted metal. Two wooden buckets filled with metal balls are on the sides.
Level 3: the stone on the tower is replaced with wood and the crossbow is replaced with a cannon.
Level 4: the wood on the tower is replaced with stone with blue flags on it. A third bucket is added and the sides of the cannon are protected with an extra layer of metal.
The best use of this tower is to place it strategically with support of other defenses to make tactical kill-zones.
Pairing it up with an Artillery Gun can provide nice power. The Anti-Tank Gun attacks incoming Heavy Cavalry that the Artillery Gun is weak to, while the Artillery Gun destroys infantry that get too close to the Anti-Tank Gun. Keep in mind not to place them too close as the Catapult/Mortar has a blind zone as well as both can be taken out by a few Barrage tactics.
Pairing it up with a Stronghold works as well, as the Ballista/Cannon/Anti-tank Tower will protect the Stronghold from Heavy Cavalry, and the Stronghold will protect the Ballista/Cannon/Anti-tank Tower from Heavy Infantry, Ranged Infantry etc, although this strategy can be easily thwarted with the Sabotage tactic.