Upgrade your buildings when you get the chance, for it's always good to strengthen your force.
Bear in mind projectile towers don't generate new troops, so you'll need to reinforce them manually.
Don't leave important bases undefended unless you're sure your assault is going to finish the game.
When you are in the store menu in between stages, make sure to click on the “Get Gems” button once every single day. Every day, you can get 100 free gems from there, but you won’t know unless you go there because it’s not widely advertised.
Once you unlock the archers and build up enough gems, level them up massively in order to protect against a whole crowd of attackers destroying your mushrooms in front of a castle. Send in one or two shield mushrooms, after you unlock them, for protection and to make for easy targets.