Primitive

I want to take mushroom cave on ragnarok, the underwater cave but I don't know how I'd do that without turrets... it's a primitive thing so no turrets or tek so I'm left sitting here wondering how I'm meant to secure the entrance. Want space to get tames in and out, I mainly wanna be an aquaticish person with all my cryptoclidius and such. There's a mod that adds an underwater dragon and I want that, which is why I want either an underwater cave spot or a base near the ocean but still defendable, asking too much I think. Oh there's plant turrets