Not one step back!
I played a number of zombie games, and defense games, but this was an interesting mix of both genres. What I especially liked is the "strategic" side of this game, letting you setup your men in whatever way you like, giving you the choice of a decent number of unlocks, and your own weapons. There are however also a few downsides...
Let us start off by saying that the graphics and sound are good. The controls are simple and easy to use.
The different games modes add variety, whether you want to play the story mode (where the story itself and the reports/notes you receive are very interesting), the challenges are decent, and of course survival is also a very interesting mode.
The choice of units and unlocks is very wide, and all of them have their uses, to a greater or lesser extent.
There are downsides, however. I finished the story mode in one go, to somebody who is experienced with Defense style games this one might not be too much of a challenge. In Story Mode the essential key to success is getting your trench filled up with a mix of riflemen and machine guns, aiming at getting both German riflemen and Machine guns. If that will be the case, and the men are properly spread out the zombie will pose absolutely no threat, and you will often just have to look where the zombies have closed half the distance (if that happens at all).
The Artillery although powerful has the huge downside of not being at all useful. And this is due to the fact that zombies often come alone, so any artillery barrage will be centred on a single zombie, which means that you might as well shoot it with your own rifle. There is a choice of guns but even the basic one will do the job "just fine" as long as you aim for headshots, and do not panic. You might as well just damage a zombie to let your own men finish it off.
Survival Mode is much more demanding and interesting. The Story Mode acting as a good "Tutorial", with an interesting story, and letting the player get used to the control, artillery and different guns. Achievements help out with giving the necessary funds for upgrades, but I feel there could be more of them.
Essentially, what I would change is adding perhaps more zombie types, and testing the usefulness of the different guns and artillery call-ins, as in the Story Mode some of them become near worthless. Perhaps adding a "Hard Mode" for the main campaign would make it an interesting replay, with more zombies to try and defeat, and the horde actually feeling like a horde, than not.
An interesting addition would be the use of "Morale". If it falls low enough your men would randomly flee, or be less effective in combat. In turn, what would influence Morale is the number of people that died in the previous missions, as well as the mix of "Nationalities" (making overall Morale lower if that is the case). Upgrades that improve the trenches would also make an interesting addition, if the zombie hordes were bigger. Adding an "officer" character that would add bonus to both morale and combat effectiveness, but himself having a poor weapon, would not be a bad idea. This would of course only work if there was a much more extensive "Campaign" mode.
Overall a good defense game, which I enjoyed. Not too demanding, but at times a bit too easy. I would be interested in seeing a sequel.