The Mortar card is mostly used as an offensive building similar to the X-Bow. It is a cheaper alternative with slow hit speed. It can also counter swarm troops because it deals splash damage to ground units.
The Mortar has a long range of target area. However, it has a non targeting zone in a circle around itself. It means that it cannot attack the enemies once they are near the Mortar, making it vulnerable to melee troops especially the Hog Rider.