By April, the land had been cleared and a storehouse built, but the French discovered the project and sent a large force of infantry and artillery to capture it.

[6] Mercer chose not to build an outer wall, but instead constructed three barracks, the commandant's house, and a storeroom facing the gate, with four diamond-shaped bastions between them, at each corner.

[1][7]: 113  This was the venue for a number of important conferences, the first of which took place on July 4, 1759, and included George Croghan, Colonel Mercer, William Trent, Andrew Montour, interpreter, Lenape chief King Beaver, and Seneca war leader Guyasuta.

[8]: 172 Due to the severe cold, construction was painfully slow, as the frozen soil was difficult to dig and felled trees had to be dragged across the icebound river.

On December 19, Mercer wrote to his commander, Colonel Henry Bouquet: "I expect in four Days to have the Place made capable of a tolerable Defence, and am fully determined to maintain the Post, or at least, make it as dear a Purchase to the Enemy as possible.

On January 3, Mercer wrote to Bouquet that "Nine Hundred and Fifty men may be contained in the Fort by building Barracks opposite the storehouses, and hutts might be raised in the front within the Intrenchments you have Directed to be made, to lodge 100 more."

An additional row of barracks was built along the shore, to house the hundreds of men needed to construct the massive Fort Pitt.

[7]: 148 In April 1759, General Henry Bouquet asked Mercer to relocate the fort to higher ground, and sent another British engineer James Robertson to assist him.

The engineer recommended a hilltop in what is now Chartiers Township, Pennsylvania, which Mercer agreed was "strong, convenient, heathfull, and pleasant," but noted that they would need long chains "for drawing up water, wood, etc.