Empty tower: click a coin in the shop, then click a slot to place it.
Upgrade: select the same coin again and click a matching tower to raise its tier (up to III).
Fuse: select a different coin and click a tier-I tower to fuse two effects into one hybrid tower.
Sell: tap a tower to select it, then press Sell (or right-click on desktop) for a 50% refund.
Leaks: enemies that reach the end drain cash straight from your balance — the tougher the enemy, the bigger the hit. Hit $0 and you're liquidated.
Between waves: whatever balance you're sitting on gets auto-invested. The market may reward you... or not.