Sentence:  Their sides present the well-known appearance of the Roman <WSD>"columbaria</WSD>" (dove-cotes), but with the important difference, that they are adapted to contain coffins instead of urns, the holes being about 2 feet square and 6 feet deep. 
Meaning of the word marked with <WSD> tags:  A large, sometimes architecturally impressive building for housing a large colony of pigeons or doves, particularly those of "ancien regime" France.