Okay, got a few things done today. I've made a few of the interior floor tiles, which have special tab-flaps glued onto them around the edges. By folding one of the flaps upwards, you can slot a support beam down over it, and it is held upright in place on the floor. By putting in two support beams, the walls (which have notches in both ends) slide down to clip into the two beams, holding the wall in place. It works really well, and I was surprised how stable it was.
I wanted the doors to be able to open and close, so I came up with this method: Get a second copy of the wall that has a door drawn on it, and cut out the door and doorframe. Notch the bottom, and notch a small piece of the thick card paper, so it can plug into the bottom of the door and hold the door up. Cut the door out of the assembled wall to make an empty doorway. Because the cut-out door includes the frame, it balances perfectly against the empty doorway, and doesn't fall over. I like the idea of 'sliding door', so to open the door, I just slide it to one side of the doorway. That way, if it's a little unbalanced at all, it won't fall over because it can flop against the wall.
Lastly, I did a wall with windows in it as well today