Built a marina inside the entrance to a Troll Cave on a small island before exploring a huge landmass. It is indestructible. Just prior to finishing "The Horde is Attacking" event happened. The only enemy to make it to the island was a Berserker, which I easily dispatched with my bow from atop the Troll Cave. Next time I find one like that I am going to build on top of the Troll Cave.

https://steamcommunity.com/sharedfiles/filedetails/?id=2697570474