Like Novi Sad and Belgrade, Nis has a citadel, but the difference is that this one was completely built by the Turks, giving it a significantly different look – note the Arab inscriptions on the gates. Roman remains were also found here, amongst others a collection of gravestones.

Other sights include Oslobodenja Square, the city's only remaining mosque at Generala Milojka Lesjanina, the people's theatre and the market that's held daily near the citadel and the bus station. Also interesting to see is how the army barracks have been rebuilt, while the restaurant next door still has a huge gap in the wall.Not far from Nis is the spa town Niska Banja.
Skull Tower

Osmanlian Turks have carried out unseen brutalities, and indeed bestialities, upon Serbian population. One of the more common penalties for Serbs was impalement on stake.The castle of Cegar (or Cele kula, as the Turks preffered to call it, implying that its building blocks were bald heads or sculls) in Nis is a unique monument in the world. It was built by the Turks into which they inbuilt the sculls of the Serb commander Sindelic and his heroes with the intention of frightening the Serb population. The message it was intended to convey (in Turkish mentality) was: to herald to all the Serbs who decide to stand in the way of Osmanlian Empire's interests (Islam) will have premature deaths, like the Serbs of Cegar.Only about 60 out of 952 skulls remain, but it's still an impressive sight. .

Nis Fortress
Nis Fortress is a complex and very important cultural and historical monument - an edifice that dominates the urban nucleus of the city. It rises on the right bank of the Nisava River, overlooking the area inhabited for longer than two milleniums.
The extant fortification is of Turkish origin, dating from the first decades of the 18th century (1719-1723). It is well-known as one of the most significant and best preserved monuments of this kind in the Mid-Balkans. The Fortress was erected on the site of earlier fortifications - the ancient Roman, Byzantine, and later yet Mediaeval forts. The Fortress has a polygonal ground plan, eight bastion terraces and four massive gates. It stretches over 22 ha of land. The rampart walls are 2,100 m long, 8 m high and 3 m thick on the average. The building stone, brought from the nearby quarries, was hewn into rather evenly-shaped blocks. The inside of the rampart wall was additionally fortified by a wooden construction, 'santrac', and an additional bulwark, 'trpanac'. On the outside, the Fortress was surrounded by a wide moat, whose northern part has been preserved to our days. Beside the massive stone rampart walls, the southern Stambol gate and the western Belgrade gate are pretty well preserved. Partly preserved are the water gates , while the northern Vidin gate and the south-east Jagodina gate are preserved only in remains.