A ladder. Specifically it's one to get out of the water at high-tide.

It's on the Hilbre Islands, England. During low tide you can walk there. But it's at the mouth of an estuary, so it can get dangerous once the tide comes. And this ladder is near an old lifeboat station

More pictures, including the lifeboat station and ladder:

https://commons.wikimedia.org/wiki/Category:Hilbre_Island

I'm no entomologist so I don't actually KNOW but I'm gonna guess outer support for a tunnel or mine shaft 🤷