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.
Although known for being transported by chariot or man, the Ballista Tower is a reference to the weapon: Ballista. The Ballista was an over sized crossbow able to heavily damage enemy cavalry, other enemy weapons and/or ships; of course with one shot with this to a man's body would result in an instantly fatal blow.
Some facts state that the Greeks stationed Ballistas for defensive use against enemies, but were mostly used upon attacking (hinting to the Ballista troop) a.k.a sieges.
Known for being used in 'The First Invasion of Britain' and 'The Siege of Alesia', the Ballista was replaced by the onager. Some armies did still use Ballistas for sniping enemies though.
After China discovered gun powder, the Chinese made the first cannon. Soon, Cannons made their way into Europe. Cannons, mostly used for sieges (hinting to the troop Cannon), were also stationed in positions to protect against attackers.
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.