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), the Cannon Tower (at level 3 and 4) and finally, the Anti-Tank Gun (at level 5 and 6).
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.
The wisest use of this tower is to place it strategically with support of other defenses to make tactical killzones.
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.