A thin slab of granite covers the ground. There are numerous cracks in it, some patched with mortar.
This is an example of a fortification created by the fortify command.
Here is how I think I created everything related to this seal:
build a hole to a flooded crypt
lock the hole
paint the hole brown
go hole
(from inside the flooded crypt)
paint location
material it as _dark_
material the hole as _glowing_
go hole
(from the deepfood forest)
fortify the hole
rename barricade to stone seal
paint seal to feldspar
into the flooded crypt
rename farbarricade to stone seal
material it
(I added _glowing_ to it)
Now here is what players can do:
Use the battering ram on the seal (to damage it and eventually reveal the hole)
Use the mortar on the seal (to repair it and hide the hole).
Damage and repair affects both stone seal objects (here and inside the crypt).
Note, the hole is only shown when the seal's health is 0. Repairing it hides the hole but if you want it well fortified you can use the mortar over and over until the seal's health is 100%.
Builders may wish to create NPC's that roam around, repairing walls and lids and things.