Start at the easiest area to defend. Even if you do not have the resources to build a lot of towers or upgrades, you can plan for the future. Use an area that covers all possible enemy troop entry points. Bring together your troops from at least two towers. Begin with one or two standard swordsman towers with a few open locations at the back. This allows to build archers or mages there later. The swordsmen and your Hero slow will slow the enemy troops to allow the archers and mages to hit them from a distance. Consider specializing your troops and not mix them. It is more important to have proper location and to start with a good tower setup early. Continuously cast spells continuously, especially during later levels. The more you use them, the better the results. Consider their cooldowns and use them with proper timing, especially the yellow spell which helps eliminate enemy troops. Use it to destroy archers or flying units first, when you do not have archers or alchemists to counter those types of attacks.