While the game is meant to be challenging, sometimes it felt rather punishing than challenging.
These changes should smooth out the difficulty, especially in the beginning.

• added 1 gold per second on ALL levels, to avoid situations when you're left with nothing
  
• Increased spawn rate of knights on level 20, from 10 seconds to 9 seconds
  
• increased base gold on level 27, from 15 to 25, allowing smoother start