Walls underground question

[Robi]

So I found a un-claimed cave nearby my hermitage and as I was beginning to move a mining alt in, I noticed I couldn't efficiently build walls on the outside due to the cliffs' northern tip being too close to water. So I wanted to build inside the cave when I found out I can't build a cornerpost on the walls of the cave, so what would be my options (if any) to secure this cave?

[NOOBY93]

So I found a un-claimed cave nearby my hermitage and as I was beginning to move a mining alt in, I noticed I couldn't efficiently build walls on the outside due to the cliffs' northern tip being too close to water. So I wanted to build inside the cave when I found out I can't build a cornerpost on the walls of the cave, so what would be my options (if any) to secure this cave?

Please Ask Simple Questions Here thread next time, please.
Anyway, in caves, there's a set amount of tiles you can't build/mine in, and after those you can. Find the first tile you can build in, pave the floor there and build a wall. Voila, secured cave.

[Robi]

Thanks man.

[SiO2]

Actually yes, in the caves there a some square beyod any cave entrance where you can not mine. But also cave ground is unpaveble closer than 2 tiles to any unmined tile. So, you should find unmineble border, clear as much space as you need and build your wall and mining camp.
closed cave entrance looks like that: