To start off:

Wall 1 > Tower 1 > Ballista 1 > Tower 2 (skip if not using mana) > Wall 2

> Cannon 1 (or Moat 1 > Ballista 2 > Cannon 1)

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Next:

Cannon 2 > Moat 2 > Tower 3 (if using mana) > Wall 3 > Cannon 3 > Ballista 3

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When done with the above upgrades:

Buy the Droid Archmagus Trap, followed by the Interdimensional Crossbow Trap.

 

Tower upgrades help to speed up mapping.

Wall 4 is a luxury upgrade; never necessary but good for farming dawn dust.