I like tower defense games, so figuring out strategy with just 3 towers was pretty easy. I do agree that for jams and demos it is good to keep things easy, and we often do that as well. Adding a push is something for later development. We started on a tower defense game ourselves while ago in Godot 3, though never finished it, which helps with thinking about how they should work from a coding angle. You have a good start. I look forward to seeing how you develop the mechanics.