@malin i don't know what technology/magic level you are dealing with. but one thing they came up with IRL was seismic sensors to look out for anomalous large movements of vehicles/people. then sending patrols out to investigate.

something cheaper would be random trip wires connected to flare guns, although that depends on how expensive gunpowder is. and probably prone to wild animals setting it off. although if there are a lot of monsters there probabaly aren't wild animals milling about.