Very well made game! Here is my honest review:

Pros

• Nice background music and VFX
• Art suits the game
• Fun sword swinging mechanic
• Very vast skill tree with various upgrades
• I really liked idea that instead of tower hp you have timer that decreases on hit
• A lot of types of enemies
• Nice progression curve

Cons

• For me player movement towards mouse felt a bit inconvenient. My player always goes into one direction farther from castle and if I want to return I just lose spinning force
• Rounds too short

My suggestions/improvements

• It would be really good to control player with WASD instead of mouse position
• Mechanic ideas: 1. you can add small spinning swords that spin around you. They appear every X seconds and then disappear after X seconds. 2. you can create slam on RMB: player jumps and when hits the ground he damages all enemies in an area and creates area that slows enemies. 3. Passive powerups that sometimes appear on the map. They give some powerful abilities for short duration. Examples: fireballs(lightning) from the sky, crazy (too fast) spinning force of sword or some other. So main point would be to risk castle to pickup those powerups for greater reward.

In overall I really liked your game. I hope you will continue to manage it further and release a full game. If you didn't understand something from what I have said, feel free to ask me.