Plovdiv was a Thracian city before it became a Greek city around 340 BC. (See the Wikipedia Plovdiv page for more history.) There is an impressive Communist-era concrete monument in Plovdiv which I only saw a few weeks before we moved away. I went will Lillian and Seth. The OpenBuildings page on Memorial complex Hillock of fraternity describes it this way:
The “Hillock of fraternity” monument symbolizes a Thracian hillock. It reminds of a stone wreath from above. A 90m long sculpture composition inside the monument is dedicated to the Liberation of Bulgaria from Ottoman rule, the The Unification of Bulgaria, the Bulgarian partisan movement and “the victory of socialism” in 1944. Bones of partisans from Plovdiv region were placed inside the memorial complex, once it was finished. It was inaugurated on 9 September 1974 by the party leader Todor Zhivkov in honor of the 30th anniversary of the “socialist revolution” in Bulgaria. There were plans to connect the “Hillock of fraternity” memorial to the Soviet army monument [colloquially known as “Aliosha” on the nearby hill Bunardjika / Бунарджика], through a spacious boulevard which would then be used to perform “festive rituals”.
It's a neat place to visit. Some of the statues have the same aesthetic as a certain style of Nativity scene, which I'm sure some of its creators would be unhappy to hear me say. It is locked, presumably to try to reduce damage and theft of things inside.
It is right next to the new Mall Plovdiv:
Further photos and explanation of the area are in these three websites: Forgotten Monuments From the Communist Era in Bulgaria, Commie Travels' Bulgaria page, and Nikola Mihov's Forget Your Past.