this is actually nice, it take some upgrades to get it going, but it is fun to play
but it has some problems, the aim is off, the cannon does not follow the mouse properly, if the mouse it too close to the tank, the bullet will hit the tank
you should add some visual to the firerate, there is no way of knowing if the tank can shoot or not, so you end up pressing the button a lot, just add a little circle above the cannon for a reloading animation, this would be great for enemies too, as they just shoot and you have no idea when it will come out
also enemies can spawn very close to the player, before choosing a spawn point, mark the closest point to the player to be unusable, this way no enemy will spawn over you, and enemies also shoot as soon as they spawn, making impossible to avoid being hit
if you could make a city map, that you can move around and hide from shots behind building, that would make the game much more interesting, depending on how much you know unity you can expand this project a lot