Mortar

Mortars (aka incendiary mortars) are unlocked via the workshop. They deal harassment damage.

Main use
They are used to set fire to the opponent's fort, which slowly spreads until repaired. If enough mortars are used and fired at the same spot, the fire can't be extinguished, since the fire is reapplied constantly.

Mortars can easily be rushed, they are pretty fast to build, you can expect them 90 seconds after the game starts (because of the workshop's build time).

They can be defended using any AA.

They can be upgraded into heavy mortars.

Other uses
Mortars deal a small amount of damage, which is enough (with the help of the fire) to destroy exposed weapons.

Using them with firebird gives them a bigger fire radius, which helps even further for harassment. This is a known strat amongst top players.