New Windsor was a new town around a new Castle in 1070. There was an ancient mill roughly in the position of the modern lock.
1681: Either in place of the mill or adapting the existing mill was built a water wheel powered pumping engine to supply water for the castle. It was known as "The King's Engine". Without a weir the current was much stronger than today, so the natural drop of the river around that large double bend was enough to drive an undershot wheel.

In 1675, [Sir Samuel Morland] obtained a patent for a certain powerful machine to raise water. By the strength of eight men it forced water from the Thames to the top of Windsor castle, and sixty feet higher, in a continual stream at the rate of sixty barrels an hour.
This was repeated in 1681, before the King, Queen, and Court, at which time his majesty presented his Magister Mechanorum with a medal, having his effigy set around with diamonds.

Bird's eye view of Romney Island before the lock
from the Eton Book of the River

1718: The old building was replaced by "The King's Engine House" - a much grander affair designed by Sir John Vanbrugh.
1742: Map showing the King's Engine and Romney Island in two sections. Lower Romney with a small bridge over the cut below the King's Engine. And then Upper Romney almost cut off by an inlet across the island. And then above that where the Cobler was to be extended a line which may indicate piles, perhaps to divert current towards the Pumping station.

Romney Island showing the King's Engine, 1742

Towing upstream at Romney Island
Seen from Windsor Bank, towing in what is now the weir stream

1768: Pictured below in a detail from Paul Sandby's "Windsor Castle from Datchet Lane on a rejoicing night" is this picture of the Vanburgh pumping house with the footbridge over to Romney Island. The big wheel can be seen within what looks like the east window of a church

King's Engine Windsor 1768 - detail from Paul Sandby painting

1794: This next picture of Windsor Castle clearly shows the Water Engine which is a large undershot water wheel. By 1794 it would probably have been, or about to be, affected by the construction of the new Romney Lock on its channel.

Windsor Castle with a clear view of the 'King's Engine', 1794

1794: The Water Engine from the above view

Windsor Castle Water Engine, 1794

1794: And in that very year John Rennie made a survey and reported:

From Windsor-bridge to near the head of Black-pots Island, there is one continued shallow, and the water is very rapid, moving nearly at the rate of three miles per hour.
The water which turns the water-works for the supply of Windsor -Castle, and also for the town itself, is taken out of the river at this place.
I measured the quantity, and found it was no less than one fourth of the water in the river after a flash had been nearly spent.
Such a deprivation as this can scarcely fail to produce shallows, particularly where the fall is so great.
Could it be practicable to take away the castle, and town's water-works, and turn all the water into the navigable channel, this alone would greatly improve the navigation.
In place of these machines, a small steam-engine might be established, which would compleatly supply their places without affecting the navigation;
but as I suppose this cannot be brought about, the only remedy I can advise, without a side-cut, is to contract the stream at the head of the Black-pots Island, by shutting up the channel on the Buckingham side, and joining the upper and under islands together.
This would swell the stream upwards, and thereby deepen the water:
but whatever is done, the section of the King's engine-stream, and the Windsor water-works, should be carefully ascertained, in order that no more water than they now possess should be given to them from the river by such works as may be devised for the improvement of the navigation;

Towing upstream at Romney Island
Seen from Windsor Bank, towing in what is now weir stream

[ See Windsor Bridge for the complex towing arrangements above Romney Island, before the lock was built ]
1774:  Poundlock decided on just above Windsor Bridge.   Materials and tools collected;  but the scheme fell through due to intense local hostility.
1797:  Romney Lock opened.  There was still at first no weir.  When a weir  was built it had opening tackle “so as to admit the largest barges navigated on the river to pass through”
1799:  Navigation “was found inconvenient from Windsor Bridge to the entrance to the new cut at Romney for want of a draught of water into the cut”

1864: Eton College -

the works are so dangerously ruined that the College gave warning that the Commissioners would be held liable for any loss of life.  The whole might go away suddenly at any time.  One of the most important weirs in the river.

1893: The weir broke down in a great flood

2011: Once again the river is harnessed to provide power - but this time in the form of electricity. A £1.7 million hydroelectric scheme installed at Romney Weir thought to be able to produce 1.7 million kilowatt hours per year. There are two 40 tonne Archimedes screws.