Not completely on topic but a form of raid upon the goblins with the aid of an actual possible bug or missing code:
Each goblin camp is made of a stockpile and a skull fire pit. Clean the goblin’s camp but DON’T let your footmen destroy the fire pit. From there, send a worker to pick the fire pit and put it somewhere in your camp. Do a fence (not too small) around the now alone goblin stockpile and you get an unlimited goblin spawning spot to train your footmen! 
If you want to end the training, just destroy the skull fire pit, that will also destroy the stockpile (and so the spawning point of goblins).
As this is a kind of exploit, I suppose the code should not allow the player to pick a skull fire place…