Could be good, but major gameplay annoyances.
I find myself fighting to defend thie little chunk in the top of the screen (unescisarrily) and that is why *I* die. On the higher levels, some of the monsters spawn almost right next to it, already making almost all of you're weapons useless besides the auto shotgun. When shooting enemies, there is also too much kick back, making the only good shooting diection down so that they wont go to the other side, making it harder to kill.
These bugs may be what makes you're game 'challenging', but if I can stand outside and not die for hours on end (as chronolized by the levels) and zombies breaking into my house will do what for the daytime? Will they eat my food?
I still beat it, but it was not a fun challenge. Games are challenges. You must beat them. Sometimes the more difficult, the more fun. Other times, the more buggy, cheap, and unpredictable, the less fun.
Other than that I really liked the gun and upgrade system. (which will have to be altered if this review is ever taken into consideration,unlikely though it is.) Ok job. Could be great. But it's not.