Buy slingers (and/or archers) to man your battlements. Attack the besieging armies, hit them with all your ammo from the walls and then conclude the sally. After 2 or 3 sallies you will kill/rout any besieging force. Also consider which cities you take to ensure that you only have a small front (so that you don't have to maintain large/expensive forces in each city, only the ones that are likely to be attacked)