This temple began as a small and primitive shrine and over hundreds of years it was built up to become a grand structure.  The interesting thing about this building is that it was built in layers.  Its core was the original building.  Because the Gods were held in high respect, the Aztecs constantly renovated their temple.  They buried the previous version of the temple with a mix of volcanic gravel and mud and then stuck stone slabs into the fill.  It was finished off with stucco and paint.  Now they had a brand new and slightly bigger temple without having to build a whole new building.  They just covered the old temple!  The Aztecs repeated this procedure seven times before their empire fell to the Spaniards.