Imperator I-class carried up 9,700 troops, as well as massive war vehicles such as AT-AT and AT-ST walkers. AT-AT barges and other landing craft were used to deliver ground assault forces to a planet's surface.
The Star Destroyer could use its guns and TIE starfighters to support any surface action. If a planet required a lasting presence, a Star Destroyer could quickly deploy a prefabricated garrison base.