Hi! Nice prototype, I enjoy all these kinds of games so I'll leave you some of my thoughts about the game:

• I would add a "loading bar" to know when the next attack of the mage will be cast (or some kind of feedback).
• I'd try to avoid too linear an upgrade tree at the beginning. Generally in incremental games we have a "clear" path to go forward, but with some ramification at the beginning you can create the illusion of choice, which would be interesting.
• I will be very careful with "no rewards runs" because it could make the game repetitive and turn boring. That is just balancing perhaps — being able to get an upgrade in every run, more ramification upgrades, or upgrades that could be purchased many times (every upgrade of the tree could be partitioned into mini upgrades to get the same amount at least).
• The game can feel very passive. I'd try to add something active, something similar to the golden cookie in Cookie Clicker — something you can point at with the cursor to get bonuses, extra coins, or something that makes the game feel less repetitive.

Anyway, good work, keep going! :)