At the end of the north nave there is the prothesis. Wikipedia: “The Prothesis is the place in the sanctuary in which the Liturgy of Preparation takes place in the Eastern Orthodox and Greek-Catholic Churches. […]
Originally, the Prothesis was located in the same room as the Holy Table, being simply a smaller table placed against the eastern wall to the north of the Holy Table. During the reign of the Emperor Justin II (565–574), it came to occupy its own separate chamber to the north of the sanctuary, having a separate apse, and joined to the Altar by an arched opening. Another apsed chamber was added on the south side for the Diaconicon. So that from this time forward, large Orthodox churches were triapsidal (having three apses on the eastern side). Smaller churches still have only one chamber containing the Altar, the Prothesis and the Diaconicon.”