Really enjoyed the game! It has vibes similar to The Perfect Tower, which is also a really good game.
Some suggestions: Many of the workshop upgrades have too little effect per level and many levels, thus requiring many many clicks. I suggest adding a bulk buy option: Spend x% of currency on the upgrade. X could be configured with an input box, or just use predetermined numbers (10%,25% and 100%).
Mastery upgrades are somewhat confusing at first, though they are easy to understand after more unlocks. I suggest showing the unlock condition for each upgrade, and add respec option (because it's a continuation of talent tree).
When dragging the game speed slider, the game sometimes reads the input wrong and the hero starts moving towards top-right corner.
I suggest showing the base HP and attack of enemies for the current wave, so that I can make more informed decisions.