I had trouble beating the 13th level (I couldn't even get past the 10th wave!) so I decided to try to get a bunch of achievements in order to rack up gems and get upgrades to help me beat it.
I go back to the final level. My plan was I would use the large amount of starting money to train a few ninjas and elves, then restart after a few rounds. This way, I would get the "Train 20 Elves" and "Train 20 Ninjas" awards.
So I start the round with a ninja or 2, and 3 archers. Remember, my plan wasn't to win, just to train elves and ninjas. To my surprise, I beat the first round, and the second, then the third, all the while training elves and ninjas.
Before I know it, my screen is full of elves and ninjas, and the message "Final Round!" pops up. I breeze through this round no problem.
I just beat the final round. No problems. On accident.
How about a text block. It would be a normal square, only it would have a letter in it. Or you could do something like the Microsoft Excel text squares, where you type in the square and text appears
 "The Man with the Invisible Trousers"){kind=icon}
 "PicTune"){kind=icon}
 "Incursion"){kind=icon}
 "Vex 2"){kind=icon}
 "Blob's Story"){kind=icon}
 "Decision 2"){kind=icon}
 "Burrito Bison"){kind=icon}
 "Music Catch 2"){kind=icon}
 "Smiley Showdown"){kind=icon}
 "Give Up, Robot"){kind=icon}
 "e7"){kind=icon}
 "Freeway Fury"){kind=icon}
 "The Codex of Alchemical Engineering"){kind=icon}
 "Neon Rider"){kind=icon}
 "Meat Boy (map pack)"){kind=icon}
 "Everybody Edits 0.6.x"){kind=icon}