So in Sonic the Hedgehog (1-knuckles), every level is essentially a giant rectangular, horizontally laid out box, and you usually begin the level in the upper left corner of the box. For most levels, this is ideal because it gives you lots of room to run left and right. However, certain levels feature more vertical action than horizontal action (Labyrinth Zone, for example). This creates a somewhat unique problem for level layout, as labyrinth zone's design calls for really high vertical chasms that extend farther than the height of the level.
So, to get around this, there's a flag that can be set which will allow the level to loop back on itself, sort of like old arcade games like ms. pacman. That way, when you reach the bottom of the screen, and continue to go down, you'll find yourself on the top of the level.
To illustrate how this translates to level design, I'll use a well known example - hidden palace zone from Sonic 2. Because this level has water in it, it's easy to show how the level wraps.
For most people, that's how HPZ ends, an impassible vertical wall. This wall is actually placed near the top of the level. If we scroll over to the right and all the way down using debug, we find more level deep underwater:
follow this underwater level portion a bit, and you resurface and can continue for a bit:
The level is actually layed out like this:
where the periods are whitespace, and the lines are platform. The trick is that the game would have wrapped around at the top of the screen and made it seem like the vertical climb was longer than it was. Labyrinth zone does similar things right before the boss fight.
Now, as you see, this creates problems with regards to the water level. The bottom of the level is submerged in water, so as we go to the top of the screen, we suddenly find ourselves dunked in water. So how does labyrinth zone get around this? By moving the water level.
As you climb towards the boss in LBZ act 3, you'll notice at one point the water level drops. If you climb back down, you'll eventually find that a door has closed that prevents you from going further back. What happened is that the game unloaded the water for the level. Notice how the water level rises back again right before the boss fight.
Anyone care to hear about how solidarity works in Sonic the hedgehog, or would you guys rather see neat unseen things?