Behind its ramparts, the Delhi citadel is more a palace than a fortress, with white marble preferred over the region's red sandstone.
The Delhi citadel is made entirely of the region's red sandstone. 
mnli hypothesis: The Delhi citadel is made entirely of the region's red sandstone.  premise: Behind its ramparts, the Delhi citadel is more a palace than a fortress, with white marble preferred over the region's red sandstone.