You can combine many boxes into an elaborate cat castle. I am so sorry for sharing this. Cut holes in between the box-floors and build cardboard ramps with scraps. Again, I apologize. More scraps can be used for fortress walls along the top. You can build wonderful, elaborate monstrosities this way. Beware this dark magic, it may consume you and those cats around you. BEWARE.

Edit: Employ the handyman's secret weapon, duct tape, liberally to make a good bond between levels. Masking tape works in a pinch. Your kids might even help decorate with marker and/or construction paper. These are the darkest of arts, tread carefully through these waters.