RA2 Mod Makers, is having a HUGE competition, with great prizes. High bridges are, of course, terrain objects that allow units to move across gorges and canyons. Low bridges typically cross rivers of various sizes, although they can be used over dry land.
Bridges consist of three parts, the bridgehead, the bridge overlay, and optional bridge repair huts.
The bridgehead is a terrain object similar to a cliff, and is only used in a high bridge at either end. The bridge overlay can be located either on the object browser or on the right-side overlay list of the terrain browser. Bridge overlay is the destroyable part used for both high and low bridges. Both bridge types may also include a bridge repair hut at either or both ends of the bridge's span. Make sure that they are at least 5 tiles in width and exactly opposite each other.