Intriguing and well-executed game, especially for the time limit! I think adding a few more sound effects for reloading the shotgun and "being in check" right after enemy movement would help - potentially a "shooting range" to experiment with the spread/feel of the base shotgun and various upgrades would be nice, too, but that's starting to get more into "extended release" territory.
I was much more scared of the White modifiers that added 6 more pawns, than ones that gave minor buffs to the King or other pieces but made for less of an encroaching wall. I think having only two choices for the next level is good, though, makes the decisions a lot more about carefully balancing details and weighing the odds of what kind of level you think your build can better handle. Three choices would probably have been information overload.
Enjoyed playing this a lot and look forward to returning to it sometime!