Essence
Spend your essense wisely. For example, when enemies are pushed up half way through your castle, summon units to help fight them directly. When a large space is opened up, keep upgrading the towers to keep it in that state.
Towers
Booby Trap towers drop sandbags when enemies are underneath them. This usually kills a single enemy and blocks other enemies from advancing for a few seconds. Because of this behavior, consider placing a booby trap at the bottom level, then position attacking towers above it, such as the Fireball tower. Place your towers properly. When the Rocket Tower becomes available, it can be used to shoot rockets at distant enemies. Position them near the top floors of your castle.