I just remove the inner ramps as well.
A few things to remember with that though. One, you can't remove the ramps directly under a bridge. Or at least you can't in older versions, haven't tried in a while. You have to remove the ramps first and then build the bridge over it.
Two, due to the way the game decides which tiles to mine first, don't designate all the points you want channeled at first. The game starts in the top-left corner of the map and works down from there, clearing a square of a certain size (don't remember how many tiles) before going to the next one down. So if you designate the entire area at once your dwarves, when they're doing the top and the bottom parts, will go all the way to the top of the map, clear a few tiles, go all the way down, clear a few tiles, then go all the way up for the next section. Wait for the entire top or bottom of the square to be channeled or cleared before designating the other part.
Three, walls on the surface can only be built a certain distance from the map edge. Starting on the very edge move over 5 tiles. That's where you can build them, if you don't want archers attacking you. So you can build a two-tile ditch and have three squares of the map left over.
Four, if you use a drawbridge (they're considered walls when raised, so block arrows) make sure the lever to activate it is close to, if not inside, your meeting area. So you can raise the bridge quickly.