The Persian king had two royal residences, one in the mountains to the north and this one on the plains further south. The term citadel wants us to think of a fortified palace. The king migrated between his two residences depending on seasonal temperatures. During the winter season (which includes Chislev), temperatures in the mountains dropped to uncomfortable chills. Conversely, during the summer months, temperatures on the southern plains were unpleasantly hot. The events of Esther also happened in Susa the citadel (Esther 1:2, Esther 1:5; see also Daniel 8:2).