Still it seems a little military muscle should also be involved in keeping the mobs at bay rather then just expenditure of candles, I'd introduce an alternative concept I'm calling 'monster-eggs' which is basically that monsters spawn not all at once but gradually in little random 'infestations'. Take the spiders for example at first a few webs spawn, then when they reach a high enough density they spawn a cocoon, then full blown spiders come out of the cocoon, then giant spider queens... If you clear out the webs often enough it never develops any further, only your military destroys this stuff so you need a military to keep things safe and your military size is determining how large of an area you can keep secure. Ideally we would have a way of assigning a certain amount of military to do these kind of search-and-destroy activity, probably through an item like the rally-point flag but which disperses them along a patrol route rather then concentrates at one point.
So what (optional) solutions do we have?
