Q1) The level size was great. But during gameplay we were not able to detect walls/objects on the level. More defined objects would help the overall gameplay.
Q2) The blood bar was great in the fact of changing into a bat to get out of a tight situations.
Q3) The horde would be more exciting if they were able to move more fluid in the since of getting held back by objects or walls. Give the player some more time to recover.
Q4) A counter or time for being a bat would be a good addition. If a player had the least amount of time being a bat would tell everyone that they made it through difficult times without flying.