The concept and idea is really interesting, I hope you plan on expanding it because It solves one of the biggest issues with tower defense games. The fact that you basically have nothing to do for large periods of time. It would be really cool if you ran around "building" new towers on the left for example, or spells that you can use.
The fact that enemies which pass the tower come inside to kill you is such a great idea. If you could upgrade your hero, it could allow you to focus on different playstyles, for example going for a lot of strong towers, or facing the enemies yourself.
Visually this is one of the best entries so far, everything looks and feels incredibly coherent and polished. Great job!