Here is such an"impregnable fortress Kozelsk":.
Another question: why did the Mongols really need Kozelsk?
Others say: Batu Khan was stuck near Kozelsk, as he"fell into the trap of spring thaw.".
Kozelsk was not a rich city,
the capture of which would more than compensate for the loss of time and human losses.
And on the seventh day, the combatants who remained in Kozelsk broke through to meet their comrades, and with them went to Chernigov.
Another question: if little Kozelsk defended itself from the Mongols for as long as 7 weeks,
what did other Russian princes do at that time?
In the spring of 1239, the Mongols are ravaged by Murom,
Nizhny Novgorod, once again pass through Ryazan land, capturing and burning the remaining cities, besieging Kozelsk.
Kozelsk was located on a hill and protected from the east by the Zhizdra River,
from the west by Drugusnaya, and in the north, as if, a canal was dug between these rivers.
Chernihiv was not hurt at all- and Kozelsk, by the way, is the city of this principality(one can
at least somehow explain Mikhail Chernigovsky's refusal to help Ryazan, but he must defend his own cities).