In reply to heavy Belay:
To a certain extent the climbs they put up will determine who goes there. If most of the routes were 7a and up there would be far more 7a climbers visiting and very few 6a climbers. If there's not much to do at a wall you're unlikely to go very often.
At my local walls there's typically the same number of 6c's as 6a's. There are slightly fewer 7a's and the grades typically go all the way to 8a+, sometimes as high as 8c, so something for everyone (though I don't think the 8c was too popular, I only heard of Alex Megos trying it and he fell off).
But from memory the Castle is not a very big wall whereas my two local walls are much bigger. It's also really expensive isn't it - like £12 to get in? The two Sheffield lead walls you can get for a little as £4.50 if you go around lunch time.
I suspect the main problem is the fact in it's in central London and rental spaces are both limited and exorbitant. So the walls are more concerned about covering their rental costs than providing good training facilities. I've heard similar complaints about one of the bouldering walls down there too: lots of very easy problems and little in the middle grades.
It also sounds like there are a lot of non-climbers using the walls there these days simply as an alternative way to keep fit so I guess in terms of making a profit that demographic is probably far more wealthy.
Presumably there are good places to train there though. Have you been to Westway? Not been there for years but it was much cheaper than the castle and much bigger too with a decent spread of grades. London has produced some pretty good climbers in recent years: Jim Pope and Molly Thompson Smith for example.
But if you're serious about climbing the best solution I would say is to get out of London.