I cast the bricks for the dungeon from rubber molds of the castlemolds brand which is made by Hirst Arts. I only have three of the molds at the moment. Numbers 53, 200, and 201.
I'm a newbie at this and now realize this first dungeon of mine has a few issues. Apparently my walls would have been stronger if glued on top of the floor tiles rather than to the side, but all my pieces are put on foam board for stability so I haven't had trouble with them coming apart at the seams. I like the foam board because it's just as thick as the floor tiles. It gives stability without too much height.
It takes a lot of time to get into this hobby because the casting of the bricks seems to take forever, as well as the wait time in them to dry fully. But I dedicated a day and a half to almost nothing but casting more bricks. I've had to put it all aside for now to work on another project totally unrelated to gaming, but I now have more bricks and hopefully when I pick back up the making of my dungeon I can get another room or two done. Or at least a couple more hallway pieces. I always seem to be short of bricks. I absolutely need a few more molds.
I use plaster of pairs to make my bricks. I hear there is better but more expensive material out there, but I bought eight pounds of the plaster already so I need to use it up before I look into another material. I chip my painted pieces a lot though, almost any bump seems to knock a chip into them. I read on the Hirst Arts page where someone adds cement color to their plaster so it's dark gray all the way through and thus when chipped you don't get the stark white contrast. I'm thinking of trying that with my cheap plaster for now. I'll of course let you know how that works out for me.