The enemy now has a miner building. It mines for resources and uses these to repair damaged buildings. Once they have mined a lot of resources they will use it to buy extra units. The repair function is a slow repair, the purpose is to repair buildings between unsuccessful attacks.
This addition will make the 2nd level of the demo make more sense. The enemies you need to destroy are now guarding mining installations.
Rebuilding destroyed buildings in exchange for resources might be added in the future.