All castles in Heroes 3.

Well, provisions are available to all towns. Are you referring to upgrading to an Arch? Yes, that's where the main strength of the Castle lies. But the upgrade is quite expensive, and you can't get it quickly on most maps (with the exception of Jebus Cross, where many consider the Castle to be the strongest). Overall, the Castle is somewhere around 5th or 6th in terms of strength, in my opinion.