Never leave one of your special reinforcements on cooldown, as long as there are enemies on the screen. Yes, you might have a stronger enemy assault coming up, but you want to keep your current troops alive for as long as possible, and these reinforcements can really turn the tide of battle in your favor.
You don't have to wait for an enemy to come into contact with your defenses for a battle to occur. Tap on the tower they're attached to, then hit the blue flag icon in the radial wheel that pops up. Now tap where you want them to travel to, and they'll move off into battle. Just make sure you don't leave a lane exposed to an attack by doing so.
It's all well and good having melee troops getting stuck into the action, but you'll be helping them out a great deal by having ranged support nearby. Your warriors can take a fair beating, but they'll live longer and the enemy will drop faster if there's a rain of arrows coming down at the same time.
It takes a little while for your Rain of Fire reinforcement to land at the destination you choose. For this reason, it's important to aim this deadly firepower just ahead of the marching enemy. That way they'll march straight into the explosion, and you'll get the most effective use from this devastating source of damage.
The best way to take out heavily-armored enemies is by making use of your magical towers. Magic damage bypasses armor, ensuring that your opponent takes a good beating as they march towards the exit point. Be wary of using magic assaults against targets that have high magic resistance though.
Whether you choose to upgrade your current towers, or build new ones, is largely a question of individual map strategy and personal preference. If you're dealing with bog-standard grunts, take them on with lots of standard units and keep the defenses coming.
If you're on top of the action, don't wait for a new wave of enemies to appear. Tapping on the early-wave circular icons at the spawning point brings the next wave into play quickly and gives you a decent chunk of change as a reward for doing so.
Not sure what you're about to have to deal with? Tap on the wave icon to get a quick peek at what's about to spawn on your doorstep. Get as familiar as you can with enemy strengths and weaknesses, so you're always ready for whatever the game throws at you.