Here is a guide to your upgrades, this will help you spend your coins wisely, so you don’t end up upgrading what you don’t need. This is not a checklist rather it is a guide to help you pick the most important.
1. You should upgrade the splash damage before the single target Defenses.
2. The Air Sweeper, Inferno Tower, and Air Defense are special Defenses – they focus on a very certain use-case. When you get them, start building them. No matter what other upgrade you could do. Building your first Inferno Tower will give you more benefit than upgrading an Archer Tower; same for Air Defense, same for Air Sweeper.
3. Don’t upgrade when there are some new building you can build, because new Defenses offer more additional DPS for less Gold and building time and they also distract attacking troops.
4. Try to avoid upgrading two Defenses of the same kind at the same time.
5. Acquire Giant Bombs, because in certain Town Hall levels they put an end to Hog attacks.