Each of these maps are created from the on-disc tileset and level codes.
If you duck/lay down at certain points in each level, there's a warp to a secret part of the level.
The full layouts have shown there are TWO in EACH level.
The only exceptions are the horseback and minecart levels (you can't duck there anyway).
These points are marked by actions codes (if that's their name anyway) 23 and 25.