Bilhorod Kyivskyi
Chlenov the place is particularly important as it is: 1 ) The only drevlian fortress surviving almost intact ( with its walls , etc . ) ; 2 ) The only fortress of 10th century Rus of such preservation; 3 ) The biggest survived fortress of that epoch . 
<triplet> Mongol invasion of Rus' <subj> 1240 <obj> end time