From the Fort Pitt Museum web site:
The Blockhouse
The small structure popularly known as "the blockhouse" (in actuality a strong defensive structure outside Fort Pitt itself that provided covering fire in any attack) is not only the last surviving building of the original fort and Pittsburgh's earliest building, but the oldest authenticated structure west of the Allegheny Mountains. Dating to 1764, it was used for many other purposes during its long history. Today, the restored blockhouse, which is administered by the Daughters of the American Revolution (DAR), serves as a final link to the 18 acres that formed the first site of the city of Pittsburgh. There is no admission charge to the blockhouse.
Overview:
Few places in America can rival the Point, a small area at the forks of the Ohio River, for historical significance. The long contest (1755-1763) between France and Great Britain that started here would develop into an epic struggle between the two imperial powers--what many regard as the first truly world war. On the fate of the Point would hinge that of half of the North American continent, as well as the destiny of the republic that would come into existence in 1776.
During the mid-1700s, the French and the British erected fortifications that protected their claims to the early West and served as bases for their trade with the Native Americans. Even after the departure of the French in 1758, the area retained significant strategic value as Americans, the British, and Native Americans wrestled for ultimate control. Meanwhile, the city of Pittsburgh took root and flourished, growing into a major industrial and commercial center that engulfed the Point. Redevelopment of the area during the mid-20th century included creation of a new state park that enables visitors to experience what the Point was like during its early years.
The Beginnings of Pittsburgh:
Once General Forbes secured the Point for Great Britain, he commissioned a temporary fort (sometimes called Fort Mercer, after its commander) to replace the one the French had left in ashes and ordered that a new, permanent fortification be built of earth, stone, timber, and brick on a site just east of the ruins of Fort Duquesne. Forbes named the area "Pittsbourgh" in honor of William Pitt, the British Prime Minister who had led the successful war against France.
Almost immediately the official name of Pittsbourgh evolved into "Pittsburgh" and the fort itself gradually became known as "Fort Pitt". One of the largest British strongholds in North America (17 1/2 acres in all, 2 1/2 acres within the walls), it became the center of British military and trading activities west of the Allegheny Mountains. Though it was never attacked by the French, Fort Pitt (along with nearby Fort Ligonier and a handful of smaller outposts) was among the few British forts that successfully withstood a Native American siege during the conflict known as Pontiac's War. During the siege, which lasted from May 27 to August 9, 1763, hundreds of people took refuge inside Fort Pitt. Only when Colonel Henry Bouquet led British troops to a victory over Native American forces in the battle of Bushy Run was the siege lifted.
Fort Pitt and the American Revolution:
With the French threat gone, Great Britain could reduce its military presence on the frontier and evacuate Fort Pitt, which was proving difficult and expensive to maintain. In October of 1772 they sold the fort and its buildings to William Thompson and Alexander Ross for £50 New York currency. Which civil authority would now govern the area was unclear, though, for Pennsylvania and Virginia both believed that is today the southwest corner of Pennsylvania fell within their territory. In 1774, Virginia began to enforce its claim to be the region "west of Laurel Hill": John Connolly took over the fort, rebuilt it, and named it Fort Dunmore after the governor of Virginia, Lord Dunmore. (This name did not catch on.) Supporters of Virginia and those of Pennsylvania clashed repeatedly through May of 1775.
When word of the battles at Lexington and Concord reached Pittsburgh that month, the Virginia and Pennsylvania factions were able to work together to support the American Revolution. Conferences with Native Americans, held at fort Pitt, achieved a tenuous neutrality between the new Continental government and the Native American nations in the region. Fort Pitt was maintained by Virginia's revolutionary government until 1777, when an increase in attacks by Native Americans allied with the British and American Loyalists prompted the Continental Congress to appoint General Edward Hand to take control of Fort Pitt, which under Hand and his successors served as the Western District headquarters of the Continental Army.
Troops and supplies were gathered at Fort Pitt for the defense of the new nation's western frontier, and a series of military campaigns would venture west from the fort at the Forks of the Ohio. In addition, negotiations with Native American nations in the region continued to be held at Fort Pitt. The first peace treaty the United States ever signed with Native Americans was negotiated and signed at Fort Pitt on September 17, 1778.
Fort Pitt's Final Years:
When the American Revolution ended, Fort Pitt and West Point were the only military fortifications that the United States Army maintained. Fort Pitt would continue as a supply depot and base of operations until 1792, when its condition had deteriorated so much that a new fort, called Fort Fayette, was built to replace it. (This fort was located near Ninth Street and Penn Avenue in Pittsburgh). The old fort was finally dismantled and salvaged, its remnants used by Pittsburghers in their construction of more permanent housing for themselves. Soon the site of Fort Pitt was being used for other purposes.