The curtain wall was enclosed on all five sides by a wide, deep moat filled with water and by a tall bamboo palisade.

[4] To remain in contact with the flotilla, the French were bound to approach Sơn Tây from the east, with their right flank lying on the Red River.

The dyke paths leading to Sơn Tây from the Day River converged at the village of Phu Sa, just to the northeast of the town.

Water-filled ditches, bamboo palisades and trenches surrounded a central redoubt, and subsidiary defences hampered the approach to the main position.

From the Phu Sa strongpoint, the Black Flags had an excellent field of fire both out to the east, from where the initial French attack was likely to come, and north to the Red River.

Neither the Vietnamese nor the Chinese contingents would play any significant role in the battle, which was essentially a straight fight between the French and the Black Flags.

Courbet had told nobody the objective of the expedition, and many of the combatants had been expecting to march against Bắc Ninh, which had been occupied by around 20,000 troops of China's Guangxi Army in the autumn of 1882.

Meanwhile, Belin's column marched over Paper Bridge and through the villages of Phủ Hoài, Kien Mai and Phong, recrossing the battlefields of 19 May and 15 August, and reached the Day River without incident.

Early in the afternoon of 14 December the French advanced from the east on the Phu Sa positions, beating off a Black Flag sortie against their right flank.

Later in the afternoon Roux and Chevallier's battalions captured the forward Black Flag positions at Phu Sa, but attempts to exploit this success failed.

Liu Yongfu lost so many men in this counterattack that he was obliged to abandon the Phu Sa positions and withdraw into the fortified perimeter of Sơn Tây.

To hearten his troops, Courbet set an example of the utmost personal courage, riding forward to a position well within range of the Black Flag fire.

At 5 p.m. Donnier's Foreign Legion battalion and Laguerre's fusiliers-marins captured the western gate of Sơn Tây and fought their way into the town.

Vous avez montré une fois de plus au monde entier que la France peut toujours compter sur ses enfants.

Liu Yongfu felt that he had been deliberately left to bear the brunt of the fighting by his Chinese and Vietnamese allies, and determined never again to expose his troops so openly.

The absence of the Black Flags from the battlefield was an important factor in the defeat of China's Guangxi Army in the Bắc Ninh campaign in March 1884.