Neutral Creep Spawn

Like in dota how theres neutral creep locations that respawn after a given time.
Im looking for a similar trigger, however the creeps spawned isn't as random; i need a trigger that for designated regions, if theres no creeps already, spawn a group of say footmen and if killed after say 2mins they will be respawned. I dont want a pool of creeps to be available for spawn, just 'footmen'.

make 1 trigger to spawn them first with unit create, and count every unit with an integer. And do a wait for condition to wait untill that counter has reaches zero, then do a polled wait for 2 min, and run this trigger again in itself:)
"Iterations are humanlike, recursions are divine"

note: u will need another trigger to run this trigger to begin with

Could you provide an example please? Also how do you make it creeps can only be lured for a certain distance then retreat to their spawn position.

MadLion's method is more effecient, but until you get to that point, here is something simple.

Thanks for the suggestions guys, i got this working now :)