Before every battle, you may redistribute your upgrade points, and it is helpful to do so based on what you intend to build. For example:
- If you are doing a challenge that bans construction of a tower type, don't spend any upgrade points on that type.
- If the mix of enemies dictates that you will be concentrating on one tower type, get as many upgrades to it as possible.
- If you need defense against Flying Enemies, Reinforcements with the Spear Throw/Recurve Bow upgrade will allow you more flexibility to not spend too much on other anti-flying enemy towers.
- When a level is difficult and you try it several times with different approaches, your upgrades should also change to support each different approach.