Adding a caveat, as many of we Soldiers would say, I would like to talk about using a structure or building itself as a defensive weapon, or perhaps redoubt.

Entrance ways, to include doors, large windows, and combat damage large enough to give entry, interior objects such as bed frames, book shelves, or other large objects filled with ballast like dirt or similar material to add that much more weight, are placed behind these areas. However, careful pains need to be made so that the defenders can not only get out, but get out very quickly if they are about to be overran.

For multiple storied buildings, secondary walls, whether dry wall or other can be taken down and placed over stairs to deny offensive movement from story to story. Furthermore, nails or similar objects can be stuck into the boards, as well as other means of disallowing enemies to get up or down stairs(think the Home Alone series).

Claymores can be placed in outward fans in partially or fully destroyed rooms with either detonators elsewhere, or trip wire set up to provide an alarm that can deny the enemy the element of surprise.

Hallways can be lengthened in terms of fields of fire by taking out non load bearing walls to make a long lane of fire within the building. Murder Holes can be placed in crawl spaces, and along walls. Holes can be cut into floors that have hollow spaces below, and concealed with throw rugs, garbage, or debris like tacked down news paper.

Trip wire at random intervals. Mouse trapped shotgun shells at random intervals.

I can think of plenty of other things, but this shall suffice I hope.

-Hauser