For an easy game:
Build only magic towers(the violet one that casts bouncing magic balls). Alternate between upgrading and building the magic tower and you're good to survive for the whole game.

Explanation: Magic towers fires magic balls that bounce at enemies that in result hits another enemy up to another. Enemies grow in numbers as the game progress therefore a lot of firepower is required hence the magic towers. Also, the magic tower has the highest range but lacks bit of power compared to fire and arrow towers but it made up for its bouncing attack effect.

If you don't trust this guide then see for yourself(^^).