Thank you for the in-depth feedback. It's always helpful to know how things could be improved.
Originally I had meant to add certain types of enemies that could only be killed by the mine bullets which you discovered, like an enemy with a permanent shield around it. I had to cut it due to lack of time.
The countdown is actually a magic countdown: https://tvtropes.org/pmwiki/pmwiki.php/Main/MagicCountdown
I trust you have played Undertale :)