Upgrade or build more? The eternal tower defence question

Every tower defence match forces the same decision: do you upgrade the towers you have, or plant new ones? Spend wrong and the next wave rolls over you. After way too many lost runs, here's the heuristic I trust.

Upgrade when your towers are already hitting most enemies but not killing them fast enough. If units are leaking at the end of your maze with a sliver of health, you don't need more guns — you need stronger ones.

Build new towers when enemies survive long stretches untouched, or when a flying wave sneaks past your ground defences. Coverage gaps aren't fixed by upgrades; they need bodies on the map.

There's a cost curve too: upgrades usually give more damage per coin at mid-tiers, while early game favors spreading out for coverage. When in doubt, upgrade your best-placed tower first — position multiplies every upgrade you pour into it.