Byzantine military settlement. https://books.google.pl/books?id=Na1EBAAAQBAJ&pg=PA297&lpg=PA297&dq=Zercel+Kale&source=bl&ots=MDsFHLvkb5&sig=aUxKCz9_q_eFZXmNB1xAnOlp43w&hl=pl&sa=X&ved=0ahUKEwjf3ZG97bPSAhUBfiwKHbzQB0AQ6AEIJTAB