from scratch

I know that you are rarely short of water in wales but the higher the bed the quicker it will dry up.
A 20" high bed will dry very indeed quickly in summer. Have you a hose that can reach it.  
Worth thinking what you may be growing.
At least at that hieght you won't be bothered by carrot fly.

Also WG makes the point that it would be difficult to weed,unless you break it down into two or more beds.

You could have three beds forming a circle (For your roundabout) and have a Y shaped centre pathway inbetween.  This would allow someone to weed in safety and they could reach into each bed to plant or pick etc.