Assaulting a city with stone walls is perhaps the most difficult job for an ancient army to perform. Such assaults were rare. Hannibal didn't dare to even consider assaulting Rome, for example, let alone actually do it.

An assault like that is meant to be very tough indeed. So a 30,000 mnai cost for the best seige weapon in the game is intended to deter you from making such assaults.

In the Hellenic world (Koinon Hellenon and Macedon) starved strong enemy cities with stone walls into submission, rather than risk terrible losses by assaulting them. Macedon did not assault Athens in 268-263 BC, the seige lasted for years until the Athenians surrendered. The Romans had Syracuse under siege for over two years in 214-212 BC, they took the city only through treachery, assaulting it during a parley.