See:
- http://www.bulgariatravel.org/en/Article/Details/238/The%20Zhaba%20Mogila%20Thracian%20Tomb%20in%20the%20city%20of%20Strelcha
- Nikola Theodossiev, Monumental Tombs and Hero Cults in Thrace during the 5th-3rd centuries B.C., https://books.openedition.org/pulg/792