Platforms can be used to garrison most troops, even Machines and Mounted units which cannot be garrisoned in Watch Towers.
Any Melee, Mounted, or Ranged unit garrisoned on a platform will create a camp there, moving to engage enemy attackers.
Any Machines garrisoned on a Platform will build a mound to increase their range and will not move.
Upgrade the Platform to increase the number of troops that can be garrisoned on it.