The wall of the rotunda is 6 metres thick and has seven alcoves which are alternatively semi-circular (3) and rectangular (4). The alcove opposite the door is the most impressive and reaches the ceiling. It has a decorative red porphyry frieze and cornice and is flanked by two Corinthian marble columns of Phrygian purple. Each of the other alcoves has two marble columns in Phrygian purple (semi-circular) or Numidian yellow (rectangular). Each alcove also had three niches for statues and a small window with another seven windows placed around the rotunda walls. The pavement is the original and consists of a square pattern using grey granite, red porphyry, Numidian yellow and Phrygian purple marble.