Article: In July 1855, the allied squadron tried to go past Taganrog to Rostov on Don, entering the Don River through the Mius River. On 12 July 1855 HMS Jasper grounded near Taganrog thanks to a fisherman who moved buoys into shallow water. The Cossacks captured the gunboat with all of its guns and blew it up. The third siege attempt was made 19–31 August 1855, but the city was already fortified and the squadron could not approach close enough for landing operations. The allied fleet left the Gulf of Taganrog on the 2nd September 1855, with minor military operations along the Azov Sea coast continuing until late autumn 1855.

Question: Why did the third siege attempt fail?
the city was already fortified