We could add a jail cell for storing zombies in between levels. Then they could level up too.
Mind Control Beam
I'm thinking about adding a new upgrade called a mind control beam. Such an upgrade would be purchased from the Fort Upgrades screen and would likely be the most expensive upgrade in the game for now.
The mind control beam will sap energy from your highest energy weapon only when you have 80 percent or higher energy. It will not sap energy when you are charging your weapons. The mind control beam will have an energy bar that goes up to 100 energy points. Once it reaches 100 energy it will take control of the closest x zombie to your fort with x being the number of mind control beams you own.
That zombie will then attack other zombies in the game by targeting the nearest zombie to him. Nearby zombies will also attack your controlled zombie forcing you to protect him. Mind controlled zombies will not attack other mind controlled zombies.
Before I start work on this upgrade I will need to polish up the high score screen.