Parthicopolis was a Roman and later Byzantine city that occupied the same valley as modern Sandanski. The ruins are scattered beneath the town and partially excavated within the central park, where you can see floor mosaics, foundations, and sections of the ancient street plan. The level of preservation is modest but the in-situ nature of the remains — in the middle of a working town — gives them an immediacy that more tidied-up archaeological parks lack.
The most impressive finds from the site are in the Sandanski Archaeological Museum nearby, including a striking collection of mosaics, pottery, coins, and sculptural fragments. The park excavations themselves are freely accessible and well-signed in Bulgarian and English.