Zindan Fortress is the most evocative remnant of Lenkoran's defensive past. The surviving structure is limited, so do not expect a grand castle, but the site helps explain why this low coastal city mattered: it controlled a strategic point between the Caspian shore, the Talysh hinterland, and the road toward Iran.
It is worth seeking out if you enjoy places where history is visible in fragments. The atmosphere is stronger than the interpretation, so read a little before you go and let the surrounding streets do some of the work.