Just a thought:
Goblins can currently be isolated through wall building, something that will become obsolete once they’re able to tear down those walls.
Another way of isolating them is to build a trench:
Those marauders have been sitting in their camp(fire) for the past 30 minutes, contemplating their life choices…
My suggestion is a wave of goblins that may occasionally spawn containing an engineer. This might be complicated in terms of AI, but this unit could a) breach walls, b) build ladders up/down cliffs, and c) create small plank bridges to cross trenches. If there’s a combat unit buildup that hasn’t been dealt with, they’d all swarm in at once, giving reason not to hole oneself in.
Any thoughts? Would the AI coding be feasible?