A little help to those who want to implement this unit, I hope they will find it useful.
Siege Towers were first time build during the reign of Philipp II of Makedonia, for the siege of Byzance, and thereafter all along the Anabase (conquest of Asia) of his son Alexander the Great. They were called Helepoleis (sg. Helepolis), built with wood, high from 20m to 45m (this maximum is reach at the siege of Tyre) and equiped with "flying-bridges" and oxybeles (arrow-firing catapults, as far as stone-launching ones were named lithoboles or petroboles).
The most famous ingeneer who manage to build such impressive towers was Poleydos the Thessalian.
I think prerequisites should be an Antic Ingeneering, Philipp II had recruted a lot of mathematicians and mecanicians to improve the power of Macedonian army and achieve his reputation of Poliorcet (Poliorcetic is the Art to take cities).