This is a fairly simple trick, but it can take a few attempts to get right first time. This is a more extended and more in-depth version of the Stronghold Heaven tutorial on this to try and avoid confusion. Points to note:
There appears to be no limit of exactly how many bridges you use in your map, but it does complicate things if you do not make the bridges near the start of your map. If you would like to use this trick but not make bridges yourself, Lord Vetka has posted ten raised bridge templates in our download section you can use.
First of all, go into the editor and select then terrain menu. Lay down a strip of terrain and place a bridge on it, from the 'Rivers' menu and remove any excess terrain. I have selected the lowest raised terrain size for the purpose of this tutorial.
Now select the crop tool, and crop everything past the bridge on your map.
Once you have finished cropping the bridge, it will become floating. If the bridge was placed on a higher level than I have, it will float in the air.
You now have one opportunity to edit under the bridge. Any water or other effects underneath it must be added before you leave the editor. I would also recommend placing a block of terrain at the same height level as the bridge at each end of it, so that people can cross it.
Now press the spacebar and go into Architect mode. Then select the smallest brush size and mark in, using a texture that stands out from the current landscape, lines around the outside of the bridge.
Press ALT + G. This will make the bridge, along with possibly some other things on the map disappear. Now using the same terrain height, create a land bridge in the middle of the bridge from one side to the other. This should be done in the least amount of strokes as possible, because units may stray to one or both sides of the bridge otherwise. Then undo this and place man traps on each side of the bridge. If you cannot get man traps on the very edge of the terrain, place them as close as you can. It must however be done on both sides of the bridge. Afterwards, press ALT + G once more - done!
Here are some excellent examples of bridges over canyons in action.