Merge Bastion

Merge your garrison, deploy it on the road, and hold the line. Recruit Scouts in the barracks, drag two identical units together to fuse them into the next tier, and deploy the result onto a glowing pad where it fires on its own. Eight tiers run from Scout to Sentinel. Enemies arrive in escalating waves that roll new modifiers, and every tenth wave the battlefield changes biome. Your Bastion has twenty hit points and nothing heals it, so every enemy that reaches it is a permanent loss.