This game has a hidden level select.
The shop guy from Clash at Demonhead challenges you to fit blocks in oddly-shaped spaces! Doesn't that sound like fun?
Enter the password ZEAL.
Unused Design Room
This code is present only in the US version of the game. Use Game Genie codes 621-5FF-3B3+081-60F-2A2, then at the title screen, enter the PASSWORD option to see some kind of very simple drawing mode instead. The background is filled with brick wall tiles by default. Hold A, then use the D-Pad buttons to draw with black tiles, hold B and also use the D-Pad buttons to erase black tiles. There is no such code in the Japanese version at all.