Ah, yeah I was speaking medieval. Rome did indeed have organised city watches. I did not know about the English legislation however. I am not sure we had similar laws here in Scandinavia, but regardless, a city with PCs should also have a city guard or personal guard of some lord keeping the peace.. elsewise things might get out of hand when the PCs it

I think that in earlier settlemens, castles or smaller towns the city guard will simply be equal to the personal guard of the reigning lord. When towns grow into cities they get city priviledges and sometimes appoint a Mayor instead, and that Mayor will be responsible for setting up some sort of peace-keeping force.
The size of the army depends a lot on the type of army used, which again depends on the period. Rome used a professional army since the Marian Reforms. The English armies that fought in the hundred years war, consisted mostly of professional soldiers - even the longbowmen would probably be from peasant households, but have taken up the bow in the service of their king professionally.
The French armies of the same period consisted mostly of nobles, their squires and guards and (Italian) mercenaries.
However, both sides would probably call in peasant levies if the campaign required additional numbers, if hard pressed after losing a battle or to replenish losses. Other places in the world (Scandinavia for instance) relied much more on levies called in times of war, since a class of knights and professional soldiers only arose fairly late in the medieval period. The same countries were also among the first to later institute a national army.
So, if you wanøt to be historically correct it is very hard to set some sort of guidelines. Usually you can put bollocks to that, and simply go with the 1% rule or something
- Dan