Given the following context:

Sisters Fiona, Evelyn, and Susie Gaylord, are orphaned when first their mother goes down with the Lusitania and then their wealthy father, Major Penn Gaylord, is killed in France in World War I. Before Penn left for France, he told Fiona, the eldest, that the Gaylords have never sold the land they have acquired.
However, their half billion dollar inheritance is held up in probate for decades; Fiona complains that they have practically grown up in court. Though they have a New York City Fifth Avenue mansion, the sisters have had to borrow money to live. A French charity claims that Penn made a later will before he died, leaving 10% of the Gaylord estate to it. Though the Gaylords are now willing to give up the 10%, their real antagonist, Charles Barclay, who wants their mansion, and the choice land on which it sits too, so he can tear it down as part of his real estate development, Barclay Square. Fiona is determined not to give in to this.
Meanwhile, Evelyn has married an English nobleman, now fighting in the RAF, while Susie is in love with painter Gig Young, despite being married herself. Susanna only stayed with her husband for a few hours, but he refuses to grant her an annulment unless she pays him a great deal of money, to which of course she does not have access. When Evelyn returns home from England, she becomes attracted to Gig herself and tries to steal him away.

answer the following question:

What type of building sits on the land that Charles Barclay wants to develop? 

mansion