The valley is the last thing standing between them and everything behind it. Hold it. Classic tower defence built for one hand and short sessions.
Each with its own upgrade path, range and role. The right mix beats the expensive mix.
Routes and choke points of their own — a strategy that saved one valley will not save the next.
For the moments planning cannot fix — call them in when a wave is about to break through.
From a gentle first run to a genuine test of your build order.