Complex sentence: Around 1969, the mall's concourse (extending between the Shillito's and Pogue's department stores) was enclosed, this in keeping with the contemporary idea of a closed-in, fully climate-controlled, shopping environment.
Split into simpler sentences: Around 1969, the mall's concourse (extending between the Shillito's and Pogue's department stores) was enclosed. This was done in keeping with the contemporary idea of a closed-in, fully climate-controlled, shopping environment.