It's a fun little game to improve upon further to use for a portfolio. I would animate the characters, can be simple but needs to be more than the static frames you have now. I would also have different zombie variants. Slow bullet sponges and faster erratically moving zombies, etc. Played for a while and didn't get any money, so couldn't buy anything. But I imagine that you can upgrade things like your weapons, maybe drop things like mines or timed explosives. I would also make the play area bigger, have the camera zoom out as you move. Maybe make it a bit more free roaming instead of a static room. I'd add things in there like walls and such so that you have to navigate more and can use the architecture to your benefit. I also think you could throw on some postprocess effects like vignette and bloom, that it would improve the graphics a lot.