Create a two part build/wave round system
Create a system to switch between the planning/building phase and the mob phase.
Create a timer that gives the user 2 minutes or so to build new towers and plan for the next attack. Once to timer has run out or the player opts to skip to the next attack phase, then the player can no longer build.
Once the last enemy in the attack phase is defeated, then the user enters into another build phase and the cycle continues.