Yeo Jin Goo plays the main male lead in both.
Both have a mystery and supernatural elements.
Similiar in a sense that both drama's have two parts in them.
Circle has part 1 Beta Project set in 2017 and part 2 Grand New World set in 2037.
Reunited Words has parts in the past in 2005 and parts in 2017, however not in a set sequence like Circle.