Well-chosen music and the gameplay is really intutive and self-explanatory. Watching the horde grow is fun, and turning people quite satisfying.
I didn't enjoy the game myself overall, which may in part just be because I'm not a fan of zombies, but I think I felt that it became a bit tedious quite quickly. The first level felt too long for there being no pushback - you are just collecting people until you've filled up a bar, with seemingly no pressure. But even in further levels it felt that the challenges were somewhat irrelevant. I was never at risk of losing - the horde always absorbed enemies quicker than they could shoot me, and the enemies themselves activated their own land mines (which is funny, but meant that I didn't struggle with them one bit.)
Once this was clear it felt too much like I was controlling the mindless horde quite, well, mindlessly. But I note that others found the game really fun and it may just be a genre that I'm not familiar with and don't get much out of, so this could definitely be a case of me just not at all being the target audience.