Heavy artillery Trophy in Iron Sea Defenders

Fully upgrading a tower requires you to first build a basic unit, then pay for the 3 levels of upgrades. The final upgrade generally has 2 options - doesn't matter which you pick.

Most of the levels will require a number of fully upgraded towers to clear at 3 stars. There are also lots of enemies on levels, so having the gold coins necessary for upgrades shouldn't be an issue.

The cheapest unit to upgrade is the basic cannon, and this should be your default unit.

Farming this should not be needed, this will come as you play the game naturally.