by Magisticus » Tue Sep 23, 2014 10:06 am
Yes, this is now not possible, people used to have that kind of structure back in world five. Added to which there is now no really effective way of blocking the entrance at the top other than bilding a wall around it - so this kind of base is unadvisable anyway as the cost of the two walls is much more than the cost of building one larger wall. If you want to build underground you are better mining out an area, or finding a more open cavern and building a full brickwall.
An underground area can only be mined into if it is clear below for the size of the underground area of the minhole - 7*7 squares, so in an underground base don't leave any 7*7 square area empty (or unmined).
You should having built your wall check that it can't be mined into from above as well, best way to do this is have a character commit a crime (tresspassing, because its unsummonable) and then have them stand on each corner of your base underground and determine position above ground with a ranger tracking them. For each corner they stand in mark it above ground with a signpost and then try to build a minehole everywhere over the marked area (you don't need to pave to do this) if you can build a minehole anywhere then simply build something underground to block it in that area.
Or to be really sure build underneath a lake!