This is a pretty good base for a tower defense that could be expanded upon! I like the setting, the enemies and portals look very cool as well. Movement could be improved, particularly with walls but it's understandable for a game jam project.
The problem with the gameplay loop is that you get a lot of mana, and you can just spam the basic nailgun without much of a strategy and permanently stop all the enemies. If you do make a post-jam update, maybe add some more dynamic unit and enemy interactions to keep the player engaged, and also up the difficulty enough that the player needs to make a creatively synergistic setup rather than spamming one strong option.
That being said, I'm not very knowledgeable about tower defense games, and I myself would probably have a hard time striking the right balance.