In reply to Rob Parsons:
> I think I remember reading somewhere that the Edinburgh Uni Mountaineering Club is (or was) given access to it, on one particular day of the year.
[Replying to myself (since I am unable to edit my original post)]:
In fact, this is mentioned in the book 'High Endeavours: The Life and Legend of Robin Smith', by Jimmy Cruickshank. From the notes for Chapter 10:
"1. Robin's compilation of 'Climbs Round About Edinburgh' in the 1957-58 EUMC Journal had this to say of Castle Rock:
'The climbing here is pretty well restricted to one day in the year. Perched around the cliff are a few of the vilest of bivouac sites for camping out the night before, and when Charities' Day comes around the hordes of peasants gather in Princes Street and the Gardens to gape at great strings of colourful noise beetling about on the annual storming of the Castle Rock. By the time we get up most of the colour has all gone black, for the whole place is sozzled in Waverley smoke, but things begin to look up again when they give us the free beer.
'The route more or less follows the broken line of rocks to the right of the overhanging mass facing Princes Street, and gives a lot of very pleasant V. Diff climbing to the foot of the Castle Wall. This last barrier is climbed in a corner, which gives quite a hard pitch of severe and delicate bridging.'
"That natural line up Castle Rock - Closet Climb (S) - was first climbed in May 1958 by Robin and Andrew Fraser. ...[snip]"
It could be a laugh - and worth a night in the cells - to do this climb. Think of it as a homage to Smith.