I'm going to make a removable section for a small house.
When the top section is removed, the house should look like a ruin. When the top section of the house is replaced, it should look like an undamaged house. This way you can destroy a house in a game simply by removing the top.
Start by laying out the basic walls of the house including the window and door arches, but don't build it too tall. For this example I'm not going to show where every block is put. Feel free to make the house any size and shape you want.
Add and remove blocks so the house is a little taller on one corner and shorter on another. Next, add the ruined blocks on the tops of the existing blocks.
Be very careful here! Do not put any ruined blocks where an arch will have to go later.
A good way to do this is to put down a ruined block, then make it a full block by putting its other half on. If the full block does not get in the way of an arch or a necessary piece, then you can glue the bottom of the ruined block there (be sure to remove its top half again).
Glue the ruined blocks onto the top edge of the building.
Next, place the matching halves of the ruined blocks on top of the ruined blocks you already have down. Do not glue these blocks down! You want these blocks to lift off with the top of the house.
These will make the top edges nice and flat so the remaining rows can be glued down.
Be sure to glue the next rows onto the smooth scraped side of the ruined blocks only! If you glue the next rows to anything else then the top will not come off.
Finish the house by gluing the remaining rows on top of the house.
Once dry, you can remove the top of the house leaving a ruin.
This would also work nicely for a bridge. You could make the center part of the bridge removable. Once the bridge is destroyed you simply remove the center section and figures cannot cross it anymore.