There are many things to like about this, and previous comments already did a good job of describing it.
Here are a few things that need work:
- Enemies are non-threatening. They are simply too slow.
- Timer is a double-edged sword. It is supposed to rush players, making them take snap decisions and induce tension. But on such a big map it mostly brings frustration.
- It is also hard to find the right amount of time to give per stage. Having enemies patrol certain routes would make it easier to determine.