The Mortar can be used effectively against buildings and ships, but it is unable to attack land units. They deal greatly inferior damage to Petard, though they can attack from great range and aren't suicide units, giving them more strategic value in a greater number of situations.
They play similarly to the Trebuchet from Age of Empires II in that they have very long range and must be packed up to move and fire.
Mortars can get card upgrades to make them more effective in their combat capabilities, the have DutchCoehorn Mortars (TEAM card: 20% damage bonus) and the Russians can field Unicorne Mortars (card: Coin cost -50% (175 coin), 20% damage and training time -50%). The Portuguese have Rangefinding that increases the range and LOS by 10, enabling to attack from safer positions and reduces the need to move them a lot. The Spanish can also augment their mortars with the use of Unction, increasing the damage.
This is a list of shipments that benefit Mortars.
Click for a list of Mortar-related Home City cards
"A mortar is an artillery piece with a short barrel and a large bore that fires massive projectiles in a high arc. The name "mortar" probably refers to its resemblance to the mortar of a mortar and pestle. The high arc of a mortar's fire reaches emplacements and buildings that are screened by terrain or walls or other permanent defenses. The projectile a mortar fired was filled with gunpowder and fitted with a fuse that was lit right before firing the mortar. The artillery crew had to carefully measure how long to make the fuse for the mortar to explode at the right moment (usually on impact). This made mortars much better at firing on buildings and fortifications than on fluid groups of moving troops."