Complex sentence: The exterior features a free-standing colonnade of 28 Doric columns on all four sides that correspond to the 28 oak trees in the alley, a common feature of antebellum mansions of the Mississippi River Valley.
Split into simpler sentences: The exterior features a free-standing colonnade of 28 Doric columns on all four sides that correspond to the 28 oak trees in the alley. Oak trees such as these are a common feature of antebellum mansions of the Mississippi River Valley.