This is one of some pictures from the Diakonikon. This is a space with a dome to the left of the bema and apse area of the paracclesion, with an entrance from only that area.

Wikipedia: The diaconicon (Greek: διακονικόν diakonikon; Slavonic: diakonik) is, in Eastern Orthodox and Eastern Catholic churches, the name given to a chamber on the south side of the central apse of the church, where the vestments, books, etc., that are used in the Divine Services of the church are kept (the sacred vessels are kept in the Prothesis, which is on the north side of the sanctuary).

The diaconicon contains the thalassidion (piscina), a sink that drains into an honorable place where liquids such as the water used to wash holy things may be poured, and where the clergy may wash their hands before serving the Divine Liturgy. The diaconicon will usually have cabinets or drawers where the vestments and church hangings (antependia) may be safely stored. Here will also be kept the reserved charcoal, and a place for heating the zeon (boiling water that is poured into the chalice before Communion). Towels and other necessary items will be kept here also.