Here's a Tip I found on a Game Maker tutorial: To balance diffuculty, it goes in a zigzag, slowly getting higher. Like it goes like level 1, level 2, level 1, level 2, level 3, level 2, level 3, level 4, ect. It's something like that.

Here's a stragety: When it gets to about lvl 10, build three blocks where the wizards go, then upgrade them 'till it's level 5. One arrow tower, one catupault,, one crossbow. Then do my next stragety: Stragety 2: Build 8 blocks around the creep's goal. upgrade them until they are level 5. then do my next stragety: Stragety 3: Build blocks that kind of like go in a maze, and upgrade them. Mostly crossbows and arrow towers.