"Detinets" is a historical center of the city. It is a part of citadel of old town with the cathedrals, buildings and monuments placed here. Detinets now is a park open for tourists. So you can walk there, rest on a bench and touch to the stones of 300, 500, 1000 years age.
On this picture you can see Cernigov Collegium (1700 y.), Boris and Gleb Cathedral (1120-23 yy.) and Transfiguration Cathedral (1034 y.)