I played for some time, you can make something interesting from this concept. I think that the movement doesn't feel very good, especially when you hold two buttons, the character often moves straight and not diagonally. I guess it can be fixed by summing movement vectors or using some other code in movement. This situation usually happens when using if-else system. In my devlog you can find a comment in discussion about smooth movement. It's for Godot Engine, but maybe you can use that in Unity https://wans24.itch.io/pongus/devlog/139908/after-update-reflections-020
Also, the game lacks some impact, for example some satisfying particles when shooting and destroying blocks. Maybe, I would make shooting direction separate from movement direction. For example, movement is WASD and shooting is left/right arrows, because now it's not comfortable to aim.
And in a lot of cases you put a bomb near a big cluster of blocks, it blows everything up. After that it's easy to prevent aliens from building something, especially when you killed the builders, so the game becomes boring and you just wait for the timer to go off.