Take a moment to think about the terrain of Outpost 2. Not all formerly lava-covered terrain is at any lower height that any adjacent terrain. So, lava flowing over the walls is a slightly silly thing to think about, unless you can explain a way for lava to never leave the ashen terrain.
Also, the standard game lets you completely stop the flow of lava from a volcano for a while, until your walls get destroyed so the lava can spread further.