Castles are worthless against armies with mages?

[RainOfSteel]

Realistically if one were to besiege a City-state [...] One would be dropping boulders completely unchecked due to the fact of it not realistically happening very often.

This is on a per-milieu basis. In some milieus, it would be as you say.

1 -- You would have standard medieval armies, castle construction, and siege warfare. The entry of casters into the siege warfare environment would be rare and quite possibly very effective. It would also provoke a dramatic response if one was available. If another caster could not be obtained as a counter, or should casters have no effective counter, then an assassin might be just as useful.

2 -- Siege magic is a standard in the world. There will be a host of siege warfare related attack and defense magic, both spells, and enchanted structures.

3 -- Also, there may be mix and match versions of the above situations. Perhaps one or a few nations, or even just one group, has developed magical siege defenses, rending them immune to conventional siege attacks. Or it could be the reverse, one or more groups could have perfected magical siege attacks and for an immense price can demolish any opponent. It would almost certainly involve all standard arms race tactics with spies and assassins going back and forth in an effort to learn the secrets or kill those who had knowledge of them.