This is an average tower defense, for 1.25€ it was an OK game.
If it is above the average free browser TD, it's way below the best PC game in the genre like Defense Grid or Plant vs Zombie.
On most levels you cannot build paths with your towers and are limited to a few emplacements. And with only 7 tower types, some of which are only unlocked in the 2nd half of the campaign. This gives for little variations and low replayability.
Basically you use similar tactics on most levels.
There are a few neat ideas, the campaign plot which follow the historical events, perks, customizable towers, surprise events. But in the end they didn't add that much.
15 hours to complete the campaign on medium difficulty. Hard difficulty don't add much.
With all perks unlocked, the game balance is partly broken. And with the proper perks replaying the campaign on hard was actually easier than the first playthrough on medium difficulty.
Decent for a low price but there are much better TDs available.