Wood glue made it plenty strong...i used 2 5 pound plates on to hold the floors/ceilings while they dried.
I know it is not a perfect brick building with cracks and not straight walls...I was going for a run down 3rd world country in the middle of the desert look. I was thinking perhaps what I would do next "may" fill those voids...mortar if you will...but do not want to fill the pores of the styrofoam as well...
here's an idea. take spackle, mix tan paint in it and some sand as well. apply it in the cracks and wipe with a wet rag or wet finger. or the caulk idea would work, like obi said, cut the tip small so only a lil comes out, cut on a 45 degree angle. smooth out the bead with a wet finger.
the spackle and sand would look more natural, plus if you fill any pores, you could always wash it out, as caulk will fill them and you would be stuck with it after it dries.
personally, i would take spackle, sand , and tan paint and mix up a bit in a bowl and apply with a tooth pick or something in that nature.