I've recently been making many more of these, and I have tweaked the technique, both for speed and, I think, to give a better result.
I wanted to avoid the fiddly cutting out around the pillars in these particular sets, and also make them look better by covering up the corrugated card along the edges. The edges were simple, I borrowed Wyloch's system of hot gluing edge strip to the sides. To get around the fiddly cutting out I decided to use a material for the walls that was easier to cut, and apply a lot of the detail, such as the pillars, separately. A bit of searching around later and I found that craft foam stuff rather like mousemats. The brand I bought is called Funky Foam and comes in around A 3 sheets for £1. I bought black so that it already gives a good colour for the walls. For the pillars I simply used an old cork borer of the right diameter (easily bought on eBay) to cut circles for the pillars.
O.K. enough background, now the step by step.
The materials and tools (except the cork borer, which was just off the picture, curses)
Cut all the wall thickness off the printed floor plans. I wanted a piece like a lopsided 'H'.
Two sheets of single ply corrugated cardboard, with the corrugations running at 90 degrees to each other (this strengthens the floor plans, and reduces the chance of warping, its the same theory as plywood). the card was cut to fit the floor plans, with a 1/4 inch border for the walls.
Glue together with tacky glue and allow to dry, preferably with a weight on top.
Meanwhile, cut 1/4 inch strips of the craft foam for the walls.
Bore out enough circles of craft foam for the pillars. Remember when choosing your cork borer its the inside diameter of the hole that matters.
Glue the floor plan to the cardboard, glue stick is sufficient (and reduces the chance of warping). Add the foam strips for the outside walls.
Cut out segments for the pillars, often these can be used for small pillars in inner corners.
I added two rectangles of foam to complete the walls.
Not quite finished, there's still the edge strip to add.
Here it is, finished.