1. Take input on the page rather than through a prompt. Current implementation breaks immersion.
2. I'd like there to be some time pressure. This can be done either by the opponent making their attacks on a timer (rather than taking turns the way it's done now) or by putting an explicit countdown for how long you have to solve the arithmetic problem. Maybe you can find some other creative way to put time pressure on the player.
3. I'd like a visual indication of when an attack has been charged.
4. Music/sound would be awesome.