(SE) one wrong turn after another

It’s an archway. Wet marble bejewelled by water, it stretches up and up and over in a perfect arc. Nothing in front of it but waterfall. Nothing behind it but sheer stone. And what pretty little hollows have been carved into it. 10 pretty little hollows, large enough to hold something. Whatever could they be for? And beneath the archway? A platform, where something (or someone) might stand.