How do you get stone walls?

[Reemi-Qc]

I'm seeing everybody using stone walls but, how to you unlock thoses walls??

I only have wood walls.

[Alive]

It's researched at the Stone Workshop, which is researched in the Wood Workshop

Edit: And at the Stone Workshop, you can research Foundry, where you get Advanced versions of the production buildings, as well as Oil Platform

[Horza2002]

You need to research them in the stone workshop building

[Reemi-Qc]

Thanks