The Cathedral comprises a nave with aisles, transepts with aisles, a choir with double aisles, and an apse with ambulatory and radiating chapels. The main altar, located in the apse, was founded about 1650 by Bishop Piotr Gembicki and created by Giovanni Battista Gisleni. The altar painting of "Crucified Christ" by Marcin Blechowski is from the 17th century. Over the main altar stands a tall canopy of black marble supported by four pillars, designed by Giovanni Battista Trevano and Matteo Castelli between 1626 and 1629. Underneath the canopy is placed a silver coffin of national patron Stanislaus of Szczepanów (also Stanisław Szczepanowski) created between 1669-1671 after the previous one (donated in 1512 by King Sigismund I the Old) was stolen by the Swedes in 1655.