If you make it all the way past Stage 64, you will make it to the Big Password Entry room. You will have one opportunity to enter the correct big password. The correct password is obtained by visiting each of the special Big Password Symbol rooms that occur every 8 stages. Each box corresponds to a respective floor (8, 16, 24, etc.) If you have noted each symbol as you encounter them, then you can enter the password. Even if you only found 7 of the symbols you can still guess the missing one by process of elimination. Entering the password incorrectly will take you to a very disappointing game over screen. However, if you manage to get all 8 symbols correct, you will be treated to the ending and a secret screen that shows you how to access the Pro stages, which is another 64 floors of block manipulating madness. Here is the correct sequence of symbols:
Note that for the X68000 version of the game, the top and bottom rows are reversed.