you will have to taior your use of the peasants to which faction you use and where the city is- I agree that using a couple of peasants to garrison your major cities is a bad idea and if playing ERE you mustn't assume that garrissoning the northern bridges will protect you.

I lost thessalonica because the goths were forced out of their last city and the hordes were created [I]inside[I] my lands. As I had hun and vandal hordes all pacing around looking to get in I caouldn't use my garrison troops and ended up shipping an army I had built up for invading africa over to protect constantinople and athens (I left thessalonica as a target to buy myself some time). It has taken about 15 turns to finally get rid of the goths and I have nearlly backrupted myself doing it.