You don't want to stick a rope round the parapet, no - you'll wear the stone and your rope more than is ideal.
What'd I'd suggest is to put a sling round a bit you're 100% sure is solid, connect both ends of the sling with a screwgate karabiner (for bonus points, find one of the clever ones with a roller), and run the rope through that; if the rope then looks like rubbing on bits of the masonry, consider using some old carpet or similar to protect. If you're not confident of doing all this right, find a friend who is.
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What'd I'd suggest is to put a sling round a bit you're 100% sure is solid, connect both ends of the sling with a screwgate karabiner (for bonus points, find one of the clever ones with a roller), and run the rope through that; if the rope then looks like rubbing on bits of the masonry, consider using some old carpet or similar to protect. If you're not confident of doing all this right, find a friend who is.
HTH!