Great idea for the jam, definitely original! Unfortunately the execution leaves a bit to be desired, probably due to the time constraints of the jam.
While the visuals are incredibly good and polished, the music is a bit repetitive but fitting and driving into zombies with the chainsaw feels amazing, the game appears overwhelmingly challenging at first because of the 5 towers to defend; that is until you realize that the zombies don’t seem to do any damage, at which point the fun part of driving over zombies starts to feel useless. That’s a real bummer! :(
Also, it definitely feels like there are really few zombies! That’s probably because they are split among the five towers: if you reduce the number of towers (maybe 3 or even only one) and increase the number of zombies there will probably be more hordes of zombies to annihilate, and I assure you it will feel so good!
Another thing I’ll say is the Ai companion feels a bit dumb, tending to get stuck on walls: while that’s definitely funny the first time, it can get annoying very fast. As somebody already pointed out, it should also probably be near the player for most of the time: you can easily lose it and never see it again because of the largeness of the map.
Overall your game is a rough gem ruined by some game-breaking issues, but it is a nice start that, if improved, can become something really fun: keep it up! ^-^