An Augustinian Priory- near Hailsham- that became a country house after the Dissolution, it has a moat, fed by the Cuckmere River, that is supposed to be the most extensive in the country. It belongs to The Sussex Archaeological Trust.
There's always something extra special about old monastic buildings that were converted into houses after the Dissolution. I must try and get over to Forde Abbey again this summer...
It's run very like a National Trust property but with extra friendliness and a very relaxed attitude. A NT property would have had curators hovering in every room; here we had the house to ourselves.
I'd say it's quite common for English monasteries and castles and manor houses to have imposing gatehouses. The gatehouse is the first thing a visitor (friendly or otherwise) gets to see and they're designed to impress. Michelham's gatehouse is splendid but I know many that are grander. The gatehouse at Battle Abbey (in the same part of the world) is a spectacular piece of work that dominates the town.