It uses "scroll rates", which determines how fast a map scrolls per step taken.
By default, the map scrolls 32 pixels per tile, or basically one tile. However, you can specify higher or lower scroll rates on overlay maps to create various visual effects:
Required
Script: Overlay Maps
Tutorial
Our final map will be composed of two layers: the sky island that we will stand on, and the sky city below.
I took out most of the floors to make it easier to see what is happening.
The first step is to simply prepare your two maps. It is just regular mapping
Sky Island
Sky City
The second step is to combine them using overlay mapping.
1. We will be standing on sky island, so we will use the sky city as an overlay map. Sky city is map 4.
2. We want to place it one layer under our current map, so we set the layer order to -1.
3. Offsetting is not important, so we leave it at (0, 0). This will just draw the sky city beginning at the upper-left corner of the map.
4. We want the sky city to scroll slower, so we give it a scroll rate of 8.
And that's it! Just load up the game and you should be able to see nice things when you're walking on sky island.