Thankyou Bingley, rkay, Rhub and mav (great photo!) - just the sort of helpful responses that I expected!

The explanation about a vertical brick wall on one side and a gradual slope on the other was a great help. I'd been picturing something like a ditch or dry moat, possibly with a fence in the middle of it!

12 months ago I would have read the word ha-ha, maybe looked it up in a dictionary (but more likely not) and continued none the wiser. It's tremendous to have a forum like this to (a) fan the embers of my interest in words and (b) provide a valuable and fascinating resource for this sort of thing.

Cheers,
Marty