By 1208, the Germans were strong enough to begin operations against the Estonians, who were at that time divided into eight major and several smaller counties led by elders with limited co-operation between them. In 1208-27, war parties of the different sides rampaged through the Livonian, Northern Latgallian, and Estonian counties, with Livonians and Latgallians normally as allies of the Crusaders, and the Principalities of Polotsk and Pskov appearing as allies of different sides at different times. Hill forts, which were the key centres of Estonian counties, were besieged and captured a number of times. A truce between the war-weary sides was established for three years  and proved generally more favourable to the Germans, who consolidated their political position, while the Estonians were unable to develop their system of loose alliances into a centralised state. The Livonian leader Kaupo was killed in battle near Viljandi  on 21 September 1217, but the battle was a crushing defeat for the Estonians, whose leader Lembitu was also killed. Since 1211, his name had come to the attention of the German chroniclers as a notable Estonian elder, and he had become the central figure of the Estonian resistance. The Christian kingdoms of Denmark and Sweden were also greedy for conquests on the Eastern shores of the Baltic. While the Swedes made only one failed foray into western Estonia in 1220, the Danish Fleet headed by King Valdemar II of Denmark had landed at the Estonian town of Lindanisse  in 1219. After the Battle of Lyndanisse the Danes established a fortress, which was besieged by Estonians in 1220 and 1223, but held out. Eventually, the whole of northern Estonia came under Danish control.
Question: How many times did the Estonians besiege a fortress established by the Danes?
Answer: 2