I have to plead being somewhat indifferent: I'd always prefer to use a nice horrid green igneous rock for things like this, something rather Cthulhoid with its own murky luminesence, but I guess that's a little incompatible with 'Whitewall'... :)
Personally, for the actual topographic feature itself, I think limestone would be more than fine, yep. Even if we don't take the comparisons too far, most people have an instinctive grasp for what limestone is [the variety beggars belief actually, and it's one of the most catch-all terms around, but hey! :)] and can picture it. Also 'limestone holds water and has cool tunnels and caverns' is a thought that I'd guess buzzes through peoples' heads in association.
Again, personally, I would give a nod to Joerg's words on quartzite, however (and again, lots of people have seen hydrothermal quartz nodules or veins, or have a rough idea what it looks like), if peeps go with limestone, but fancy a variety in building stone for elements of the fortress itself.
Cheerio,
Stu.
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