Looking at them though, I wonder if they also had parapets and some sort of ladders or stairs to reach the top of them from the inside. It wouldn't at all be surprising, I think, for the archaeological evidence of the ramparts to survive much longer than some wooden parapets.
It's just... while those ramparts are very impressive, if you're trapped on the inside and can't even get up on top of them, the siegers can build ramps or ladders at their leisure. At the very least you'd want to be able to shoot some arrows or throw some javelins down at them to discourage that sort of thing.
https://www.theguardian.com/books/2013/nov/21/asterix-picts-...
https://www.asterix.com/en/the-collection/albums/asterix-and...
Another great example of this is the Phonecians, or Carthage. They likely had trade outposts in Cornwall generations before the Romans over extended themselves.
I'd love to see something about the vitrified forts (https://en.m.wikipedia.org/wiki/Vitrified_fort) the Picts were known to build. These basically hardened glass walls would have been more resistant to attack, but why they over-engineered these walls is a bit of a mystery.
Current sea level rise due to climate change is higher than that, on the order of 20cm/year