Thanks David,
I am too long away from the auld country to know these little snippets. Is it possible that Caerwall could have merely meant Big/Strong Fort, or should it have been Wallyscaer
All the placenames of forts start with Caer or Car so I suppose caerwallys would be correct in this convention.
Wallace is supposed from "welsh" but could it simply have meant "Strong".
On the other hand the expression might have come from Wallace himself who was regarded as big and strong therefore the term Wallys was an allusion to the Scots patriot.