For information:
Starkenburg does not refer to the castle itself, but to the administrative district of the Oberamt Starkenburg ("Upper/Superior Office of Starkenburg").
The Oberamt of Starkenburg belonged to the Archdiocese of Mainz, but was pledged to the Kurpfalz (Electoral Palatinate) from 1461 to 1650, which exercised sovereign rights there until 1623.
Hesse-Darmstadt did not exist in this time. It only came into being in 1567 with the division of the Landgraviate of Hesse and became the Grand Duchy of Hesse when it joined the Confederation of the Rhine in 1806.
