The northern suburbs, you can take the Kifissia-Piraeus urban railway and get off at Omonia Station, then take the Metro and get off at Syntagma Square Station. You can also take a bus to Omonia and then transfer to the Metro line at Omonia Station, travelling in the direction of Syntagma Square.
The southern suburbs, you can take the tram or the bus and then get off at Syntagma.
Piraeus, you can also take the urban railway or the bus and get off at Syntagma.
The eastern suburbs, you can take the suburban train and get off at Doukissis Plakentias Station, and then take the Metro to Syntagma Square.
The western suburbs, you can take the Metro (Agia Marina) and get off at the Syntagma Square exit, or take the bus to Omonia, then switch to the Metro and get off at Syntagma Station.