Gold is the resource that builds the board. Spend it where it compounds.
What gold does
Gold buys units and rerolls the shop. A unit you cannot afford does nothing, so every coin should either widen coverage or deepen a weak lane.
Rerolls
Reroll when the shop lacks the role the next wave needs. A reroll then is cheap insurance. Reroll when the shop already has it, and you wasted gold you could spend.
Buy the gap
Skip a unit that covers a lane you hold. Skip a tier you cannot place. Buy the role the next wave exposes. Gold on a duplicate is gold off coverage.
Compound
Spend on coverage first, then merge what you have. A board that covers every lane and deepens the weak one clears more than a board of heroes with a hole.
The unit guide shows what each bonker does, so you can spend on the right role.
Gold is the scaling resource
Income comes from clears plus any economy bonker you field. Spend on merges, not on widening the board — a merged unit pays back every wave. Early game, one economy bonker funds the rest; late game, you need two to keep up with the spike. Bank a little before the boss wave so you can drop a panic merge.
Spend on the gap
Before each purchase, find the lane that is weakest and buy the role it lacks (tank, then dps, then support). The unit guide lists what each bonker does, so you buy the right role instead of another copy you cannot merge yet. Waste gold on the wrong role and the spike walks through.