A level 1 Mortar can one-shot level 1 Archers, level 2 can one-shot level 2 Archers, etc.
At each level, the Mortar can kill a same-level Archer in a single-shot. At Town Hall 9 and above, where you get heavier defenses like the X-Bow, they are best used on the outside with trash buildings to pick off and clump other troops. Since the Mortar deals heavy splash damage, it can quite rapidly eliminate attacking light troops. It is smart to form a triangle or square with your Mortars near the center of your base, as this allows multiple Mortars to attack the same area. Mortars should generally be placed towards the center of your base, preferably behind at least one set of Walls, because its long-range attacks which allow the Mortar to attack from far behind other structures. Due to the fact that the Mortar's shell takes awhile to reach its target (over a second at maximum range), fast troops like Goblins can easily dodge its attack if they are moving when the Mortar fires. It also cannot attack air troops, such as Balloons.
A Mortar's major disadvantages are its inability to attack troops within its 4-tile blind spot, its low rate of fire, and low DPS which makes them less effective against high-health troops. It is possible that a troop, after getting knocked back, will change its targets depending on the current situation. The Mortar shell's explosion is so powerful that it knocks back smaller troops. The Mortar's low Damage per Second is due to its very slow firing speed multiplying its DPS by 5 (its hit speed) will give you its damage per shot, which is the more important stat for Mortars. Multiple Mortars can sweep away Giants and Wizards. Their splash damage, combined with their long-range, makes them deadly weapons against large groups of weaker enemies such as Barbarians, Archers, or Goblins. Mortars shoot long-range explosive shells every 5 seconds which deal devastating splash damage to every ground unit within a small radius of the impact point.
Your Multi Mortar needs to be level 8, which is at Builder Hall 8, or higher in order to gear up your Mortar. The geared-up Mortar fires bursts of three shots, albeit with reduced damage, and it directly shoots without hesitating at first like regular Mortars. Starting at level 8, which is at Town Hall 10, the Master Builder in the Builder Base can gear up a single Mortar to make it resemble a Multi Mortar. Whenever an enemy comes into range, it blasts out an explosive shell that deals area damage. Mortars are the player's light splash defenses, and is the first unlocked defense capable of doing area damage.