This gate was the official entrance to the palace proper in Tatoi (see previous posts). On the right (not visible) is what remains from the guard's outpost. From this gate would pass every official visitor and on this spot stood the tanks which blocked the palace during the coup d' etat in 1967. (That is, until the moment when the King accepted their "government").
Today the estate is completely accessible to the public (except from any building) and is recommended trip for everyone visiting Athens.