Sentence:  It [the monastery of St. Lawrence in Escorial] cost eight millions; it was twenty-four years <WSD>abuilding</WSD>, and the founder himself saw it furnished and enjoyed it twelve years after, ... 
Meaning of the word marked with <WSD> tags:  Being built or under construction, as a structure or a vessel. (First attested in the mid 16th century.)<ref name=SOED>(page=10)</ref>