High above Imotski rises the fortress Topana, located above the gorges of the Blue Lake. The fortress has an irregular shape that follows the shape of the gorge on which it was built, about 40 meters long. It was built on several levels, and the visible remains date from the 14th to the 18th century, with the earlier layers being archeologically preserved.

It is naturally protected on the two sides by a lake, and on the third side by a dry and deep sinkhole.