The limitation has been used very well, and the main element of the gameplay is solid. I'm not a big fan of this type of game, but walking left and right and shooting at opponents was quite enjoyable. The audio-visual setting also does its job.
The main downside of this game is the lack of any change in gameplay. It would definitely be useful to add something that would distinguish the first 30 seconds from the fifth minute of the game. It doesn't have to be anything complicated, in my opinion a simple scoring system or a few additional opponents would add charm to the game.
But when I think about it, a big “Highscore” sign wouldn't really fit the theme of the game, so it's quite difficult to come up with something that would diversify the gameplay without ruining the whole atmosphere.
Overall, I think the game turned out cool.