Battles of 1944 is a tower defense game set in the midst of World War II. Depending on the specific level, the player holds off the vehicles and soldiers of either the Nazi or Allied forces with an array of upgradeable soldiers, artillery, and tanks. (It seems to make no difference to gameplay which side you happen to be on, except for some cosmetic differences.)
It's hard to say anything definitively about the game rules because the application is sorely lacking in any kind of help or tutorial. Although every strategy game requires some trial-and-error experimentation to discover ways to press the advantage, Battles of 1944 is so lacking in instruction that a new player may think it just doesn't work at all. For instance, a soldier structure is far better than a tank at tackling large numbers of weak enemies, which is completely counterintuitive. The different defensive structures aren't even introduced progressively.
Still, once the interface and the basic units get figured out, it's not at all a bad tower defense game. While it lacks status effects and support abilities, it does have a manual unit aiming system for additional player control, and damage taken by the player's structures makes the game more interesting.
Sam's Protip: Better to sell and replace a first-rank structure than to repair and upgrade it, since repairs are independent of damage taken.