The wall and archaeological research campaign that began in 2014 at the eastern precincts of the Princely Palace in Alba Iulia has also reached the “D” wing, located right on the southern alignment of the Roman castrum (Apulum). Here, in 2017, the southern façade of the wing was uncovered by layers of cement and other recent materials over a length of about 40x5m. Using the methods of building archaeology (as direct observation of the stratigraphy, of construction materials and techniques, of continuities/discontinuities of the masonry, but also chrono-typology and detailed survey), a series of new data on the building stages and their chronology were obtained. Thus, it was found that the stone wall was built in opus quadratum with mortar containing pieces of crushed bricks; several blocks of stones are Roman spolia, which support the argument for a restoration of the castrum wall even in the Roman or post-Roman era. Works that can be attributed with certainty to the Middle Ages have not yet been detected, but it is known from other castrum sides research that the medieval Catholic Bishopric of Transylvania and the chapter of Alba Iulia supported repairs and constructive adaptations of the Roman fortification. In the area of the palace, the same structure of the castrum was reused in the 16^th-17^th centuries by the prince Gabriel Bethlen (1613-1629) to extend his residence to the south and east. As evidence of his period, there are brick repairs of the ground floor windows, and the window frames cut in stone with triangular pediments that are preserved on the facade. Afterward, the Habsburg army once again renewed the frames of the ground floor windows by brick (18^th century).