what You need:

Aom The Titans
New X Editor
- which you can download now---
http://aom.heavengames.com/downloads/showfile.php?fileid=1746&ci=ea3cb4dd85cf
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First up by making a river or a black rock line like this

then put down about 4/5 "wall long" on each side.
now press objects then press position objects not position object

then click on a wall and press the looped arrow and make it flat like this

do that to all of the wall pieces. now since you can't just walk on walls so go to terrain then paint land

if you are just using black rock just paint terrain.
(black rock or any other terrain like cliff is easier to make a bridge with)
now move all the wall pieces on the painted land.
now get four more long walls and move them to the sides like this

now its time for extras like wall connecter and a gate or two...
final product

and the final product with black rock

or

you can produce better scenes than this with just a little bit of imagination

Hope Youve Enjoyed my Guide By Sorceror Of Erebus