I think it's a strong basis for a topdown shooter! Here are some things I would suggest adding:
- being able to move in 8 directions would make movement more fluid
- making the jewel harder to defend over time, by for example removing a wall after a certain time has passed would make the game feel more dynamic, and would give the player a stronger incentive to continue playing
- a ton of different free to use music tracks can be found on opengameart.org, adding one would also help the game, but other comments already pointed this out
Of course, it's a bit hard to suggest "possible" improvements for a game made only within 48 hours, because the time frame is so short. So I tried to recommend things I think should not take too much time or work to implement. Great job on the jam!