I love top-down alien/zombie shooters even the not so good ones but this one is bad. An MP oriented shooter, only its unplayable.
Here's the problems:
Your basic weapon is a pistol, weak and slow. For every weapon pick up you gain two clip with it, after that you drop back to pistol. There is no weapon select even if you take that one extra weapon. The two clip is a joke especially for the railgun and the rocket launcher as these clips consist of 1 shot!.
The reload minigame is a nice feature but not in a game where millions of zombies running towards you.
The shop system is broken as time not stops not even in the SP campaign. The upgrades are even more fun as you have to go deeper in the shop's menu in real time. For your hard earned money (scrap-metal) the bought turrets last only a few seconds on normal difficulty (haven't tried harder).
The screen is too small. The charging type enemies are nearly impossible to avoid as you don't have that small reaction. The poison bomb fatty's poison area is annoyingly high and unavoidable.
The best thing that there are two enemies that can and will ATTACK FROM OUTSIDE THE SCREEN (at least on 16: 9 resolution) giving you no chance to do anything. The spitters and the megaannoying pullers are just terrible. The best happens when 2 pullers come, one pulls you out and the second pulls from the first, than the first pulls from the second and they start ping-ponging with me and i cannot do a f@@@@ing thing.
Sure it's an MP game but they were nice enough to put in an SP campaign THAT HAS 6 LEVELS! These: 1 is a training defense mission, easy / 2 you are invincible and must drive a car and kill zombies / 3 you must defense a cabin (!) that you abandon when a helicopter comes. If you wait enough and don't take the copter, you can see the bosses too and dies very fast. 4 The only mission that is good, must go through a sewer. 5 Again invincible in the copter shooting helpless zombies down there. 6 Defense mission so hard had to try it 6 times on easy difficulty. The winning screen is just an offense. Seems like a new level but it's not, it's the end.
For a real MP top-down shooter choose Nation Red, for SP choose Nation Red (yes, that's SP is very good though there is no real campaign) or Dead Horde.