there is some kind of "ramp" doodad that you can place in front of walls, but it only works at a level difference of 1 (See doodads>bridges & ramps). Maybe thats what you're looking for.
first off make sure your castle walls are wide enough for a few people to pass through, then yes, you'll either want to get a ramp from somewhere (i dont see any ramps yu would want here) or you can always build up the landscape and use the ramp feature in WE (looks like crap if not done right, but it does work).
remember that you an raise and lower the doodads in the WE, its a cheap but effective way of covering up the fact that you are using the terrain buildup to set up the castle (lower the doodads into the ground surrounding the walls, they dont see the "cliffs", only the walls will show when done right) thatt way does require more work though...
