Plovdiv of which are high. Because of these hills, Plovdiv is often referred to in Bulgaria as "The City of the Seven Hills". Plovdiv is host to a huge variety of cultural events such as the International Fair Plovdiv, the international theatrical festival "A stage on a crossroad", the TV festival "The golden chest", and many more novel festivals, such as Night/Plovdiv in September, Kapana Fest, and Opera Open. There are many preserved ruins such as the ancient Plovdiv Roman theatre, a Roman odeon, a Roman aqueduct, the Plovdiv Roman Stadium, the archaeological complex Eirene, and others. The oldest American educational
Plovdiv Airport and Frankfurt) in 2015 after two years of stable operation. The government have started the first concession procedure back in 2011, but there were no candidates for it. The second attempt for the concession was made in the spring of 2016 when the government have opened a tender for a 35 years concession of the airport, but the tender was shortly cancelled for non-compliance with European rules. In December 2016 the government opened the 3-rd tender for the concession. In the new procedure 3 companies have made the bid for the airport - Silk Road Plovdiv Airport, Consortium Plovdiv Airport