In reply to Chris Craggs:
> Despite some others have said - door downhill (so if the site floods you don't risk filling up)
In 50 + years of camping that has never happened to me (fingers now firmly crossed!), but I have felt a huge volume of water running under the ground sheet during a thunderstorm in the Drakensberg Mountains - it felt like a moving water bed! Brilliant groundsheet which didn't leak.
I also once camped in the dark, in a depression on the Purbeck Hills to get out of the wind. When I woke up, after a night of heavy rain, I found that my "depression" was in fact a dried up pond which had become a pond again, with me lying in a foot of water and the "shore" about 5ft away in every direction.......
My inflatable mattress was foating inside the tent!