Great work, good fun. Liked the zoom and the vampire buildings being too large to cover when zoomed in.
I think it needs night vision mode and to take place at night with the moon instead of the sun moving across the sky.
Liked how the vampires attacked faster and it ended when I did not hold them off.
A bit too much on rails, most on rails shooters get around this by having the players in vehicles e.g. on a train, or wagon trail. Or the opposite where you can just move around and fight the vampires on foot and if you miss one from the tower potentially take them out entering the camp.
Side note it looks like your gun model is a bolt action rifle, could add a tension if it takes time between shots and the player hears the bolt being used to chamber the next round. Also it did not run out of ammo and a reload delay could add tension.
Seemed slow in full screen mode on my PC did you used any form of occlusion culling as this ensure the renderer ignores doing the calculations for the things not in the players view.
Possible improvements a map screen (could be an overhead drone) that helps the player keep track of the vampires.