1. Whether the ability to move towers is an engaging mechanic.
Yes, the choice of where the towers are placed works well. Simple maps teach the player about simple field of fire tactics and more advanced maps can provide more of a challenge.
2. How the controls feel.
Once you get used to them you can think through problems without having to stumble over the controls. Pretty solid start.
3. Whether there are any bugs.
Haven't found one yet.
4. Feelings about switching between rooms after every wave.
The change is good, games like this benefit from a variety of levels. Perhaps it's worth playing with a couple variations in enemy forces in stages before changing to the next wave. As towers get unlocked it becomes a good chance to try new tactics and enjoy upgrades.
5. Thoughts on the passive upgrade system.
Again, a good start. Passive upgrades feel right for any gun/projectile mechanic. With development beyond the prototype stage, anything that can be done to make the upgrades feel satisfying to a degree should be explored.