 Although the Chinese and Vietnamese contingents at Sơn Tây played little part in the defence, Liu Yongfu's Black Flags fought ferociously to hold the city.On 14 December the French assaulted the outer defences of Sơn Tây at Phu Sa, but were thrown back with heavy casualties.Hoping to exploit this reverse, Liu Yongfu attacked the French lines the same night, but the Black Flag attack also failed disastrously.