Larger mortars were of two types, Land or Sea Service, which, like all ordnance of the day, were either of bronze or iron. Land Service mortars were lighter than their equivalent caliber of Sea Service mortar, since they had to be lifted manually on and off carts and into position, whereas the Sea Service mortars were installed in "bomb ketches" and needed no manhandling. They ranged in size from ranging from 4.4" to 13" in caliber. The mortar shown above is a 13" mortar meaning it fired a 13" round shell.
Mortars have about the same range as a howitzer, which is about 750 yards.
When set up and properly configured, the mortar shell's trajectory would put it over the defensive walls, and explode just above the troops sending devastating shrapnel into the ranks hunkered down behind the wall.
Cannons used at Yorktown were either made from either iron or bronze. The bronze weapons were lighter and easier to maneuver. The iron weapons were heavier and could take a much larger charge allowing them to have further range. They were also less expensive to build.
With bronze guns the metal was softer and wore down faster. They could however, be melted down and recast as a new weapon. The iron guns when they did wear out could only be scrapped.