My guess is he chopped the stairs using the spiral staircase method, raised up a ton of gozas to two places higher than where the floor would be, placed, lowered and chopped a ton of pentagrams square. Really beautiful execution.
Yup, it takes a little patience to get it just right. For example placing the penta centers around the edges are blocked by the walls. To get around that I saved the finished design, removed all of the blocking walls, placed pentagrams, then reloaded the saved design. You can lock down individual pieces of broken multi tile objects. My guess is the custom tool doesn't quite know what to do with them so it leaves them in place even in custom mode. The catch to that is the broken locked down pieces do not display as locked down when mousing over them. If they arent locked down then they are destroyed on going into custom mode. When I have alot of locked down pieces like this I always hold my breath going into custom mode to finish the design. Also, any locked down pieces become unlocked at server up. SO get it all done in one day or make sure you have everything locked down again before going into custom mode the next day. Oh and the penta centers are about half way up the first floor. Use banmboo tile pieces from the custom tool beneath them. Bamboo tile pieces force everything above them to appear when the house becomes visible. If you dont do this, the centers wont load untill you hit the steps. So the design wont look right until you hit the steps and if you are running up the steps at the time, you may get stuck dropping down before your client loads the tile.
The origianl design is long gone but the final product still exists on Catskills. I hate working in sandstone but it turned out ok: