On the list of 44 sights in Skopje of skopje.mk/ this fortress is number 43. But this number is not a ranking. The fortress is best viewed from the city, but when you go up the hill to have a closer look at the ruins, you are also rewarded by a great view over the city of Skopje and the Vardar River. The entrance is free and although some places are not reachable, it is still worthwhile to see it. There's a parking lot on the hill and from there you can walk to the ruins without climbing the hill.
"The Skopje Fortress, commonly referred to as Kale (from kale, the Turkish word for 'fortress'), is a historic fortress located in the old town of Skopje, the capital of North Macedonia. It is located in Centar municipality and situated on the highest point in the city overlooking the Vardar River. The fortress is depicted on the coat of arms of Skopje, which in turn is incorporated in the city's flag.
History
The first fortress was built in 6th century AD on a land that was inhabited during the Neolithic and Bronze Ages (roughly 4000 BC onwards). It was constructed with yellow limestone and travertine, along with fragments of Latin inscriptions. Material for the fortress originated from the Roman city of Skupi, which was completely destroyed by an earthquake in 518.
The fortress is thought to have been built during the rule of emperor Justinian I and constructed further during the 10th and 11th centuries over the remains of emperor Justinian's Byzantine fortress which may have been destroyed due to a number of wars and battles in the region. The city was capital of the First Bulgarian Empire between 992 and 1015, and was center of the uprising of the Bulgarian Empire against the Byzantine Empire under the rule of Peter Delyan. Not much is known about the Medieval fortress apart from a few documents which outline minor characteristics in the fortress' appearance.
In 1346 at the Skopje Fortress, Stefan Dušan adopted the title of Emperor at his coronation and he had transferred the capital of Serbian Empire to Skopje.
The fortress was partially destroyed yet again by an earthquake in 1963 but was not reconstructed until recently."
Source and further information: en.wikipedia.org/wiki/Skopje_Fortress