What's this for, then?  I feel the answer ought to be at my finger-tips, but not immediately so - so I thought I'd ask here.    Apologies if I'm covering well-trodden old ground.

I for one have no idea - part of the castle sewage system?    Water tank for the Railway?

Canmore has a Wellhouse Tower around that area as well as a Crane Cradle.

MB

There does appear to also be a narrow tower a short distance from the appendage.

Quoted from jmb

Canmore has a Wellhouse Tower around that area as well as a Crane Cradle.

MB

From its appearance I think the latter looks more likely. St. Margaret's Well must be quite close to being vertically below it, so maybe water was hoisted up with a crane. Supplying the castle with water must have been a problem before the days of steam, since its height would have made a piped water system impossible without mechanical pumps. And since the original road up to the castle from the West Bow was very steep, using water wagons would have been difficult.

Good job it wasn't 'The Appendage' in question that decided to go for a wee wander...

Cars damaged by rock which fell from Edinburgh Castle

Amazing that no-one was injured - it's hardly the quietest time in Edinburgh at the moment!