Pretty neat little idea! The main menu being so complete was a nice thing to see, being so rare and having so many nice details like the external links, but perhaps allocating more time to the in-game UI (making it readable even when on the already-white walls by outlining it with black, for example) or adding some sounds might have made for a more fleshed-out core experience.
The most interesting part of your game was the constant need to switch between guns. Maybe emphasize that by adding more reason to do that. Add a timer that kills or damages you if you wait too long between pick-ups. Make the next gun more powerful if you miss less shots with the previous. Make pick-up combos, where you switch quickly enough between guns to then get a rarer one spawned in.
Of course, emphasizing which gun you have would be easier with the UI change I mentioned earlier. Minimalistically speaking, you did a good job as well. : )