Really good! The concept is wonderful, and I would play this a lot with some adjustments :D
Using only the mouse is a great choice, however I would have preferred keyboard controls for the character because it was really hard to maneuver around the map and sometimes it would go around instead of going straight towards the point (maybe due to the pathfinding). A visual indicator of the path would have been good in case you don't want keyboard controls.
Searching for towers was also a bit hard, and I was afraid to explore, which is... the opposite of what I should do. I think there is a lack of delay between each waves because you jump straight into the next one. What could be cool is a portal where enemies come from, that open itself after one minute or two, or that you can open yourself. It offers greater control over this aspect of the game and enable the player to engage more in the exploration part of the gameplay.
Towers were really hard to find, at least for me. The map seems to be pretty big, and it is easy to get lost, pair that with the controls that I find a bit clunky and navigation is hard. Towers were scarce, the first one I found was about three to four waves in, and the others took way longer.
Unfortunately, my search for towers led me to lose quite a lot of castle health. A small arrow/indicator on the screen pointing towards the direction of the castle would have been a great thing to have.
With these said, I think this is a great concept that you could build upon in a post-jam version.