Last time I saw a Purge happen at the base of someone where the base was unreachable from the ground, skeletons and T4 humans spawned atop the base and started shredding it. This was a week or two ago (just after the big patch) and it was on one of those tiny “sea stack” islands around O7.
Basically, the top of the sea stack was about 100-feet above the shore and the top had been completely covered in a circular foundation. I don’t recall anything at all spawning at ground level, but by the time the purge had finished I’d say at least 1/3 of the foundations had been destroyed.
Since then, that player connected the top down to some structures at ground level and bridged it to the mainland. Following that, their most recent purge spawned on the mainland and performed a normal attack upon the structures on the shore by marching at it from a few hundred feet away.