• RNG can be strong but the game seems fairly balanced so far...

Level 1
Objective: Simply survive all waves
My rating: Shouldn't be a challenge
Notes: It's the tutorial, easy peasy lemon squeezy

Level 2
Objective: Simply survive all waves
My rating: Shouldn't be a challenge

Level 3
Objective: Survive all waves (mandatory)
Secondary: Gather 3000 gold (not mandatory)
My rating: Tough to get both objectives, not so tough to just survive
Notes: I didn't managed to do the secondary objective yet, not enough waves to get to all the gold mines... Will test again later

Level 4 - Save the Mage
Objective: Survive all waves (mandatory)
Secondary: Save the mage (mandatory)
My rating: Tough one

• Stack all your building (except walls and pylons) on the opposite side of the cage
  
• Like that, if the Boss Bat is spawning on the opposite side of the cage, you will have time to rescue the mage and come back to your castle
  
• As the Boss Bat will destroy everything it can on its way, hence losing time
  
• On the cage's side of the Castle, just pylon and walls spaced out

This was my setup for this level:

Level 5 -
Objective: Survive all waves (mandatory)
My rating: Nothing too difficult there, just play until you get it