They're called a helepolis. These are slightly post-Alexander monstrosities, loaded with catapults on the lower floors and oxybeles on the upper floors. The largest recorded was something like 30 meters tall (100 feet)! I can't believe they actually got these in the game...

Maybe they're just normal armoured seige towers (i.e. no armaments, just hide covering and a stairway for troops to climb up to the top). That would be VERY cool if they are helepolises.