The oldest church in Kalisz, dating back to the 10th-11th century, was discovered during the initial phase of archaeological excavations of the stronghold (1958-1965). In 1959, within the foundations of the apse of the church identified with the collegiate church of St. Paul, enigmatic wooden structures were uncovered. The researchers, Iwona and Krzysztof Dąbrowski, interpreted them as remains of the graves of Mieszko Mieszkowic (died 1193) and his father, Mieszko III the Old (died 1202). During the second phase of archaeological research, a team of archaeologists led by Tadeusz Baranowski uncovered and inventoried further fragments of these structures. They were found both in the apse and the nave of the church. Analysis of their stratigraphy and structural analysis led Tadeusz Baranowski and Leszek Gajewski to interpret the discovered relics as relics of the earliest church in Kalisz. It would have been a wooden building constructed using a log construction technique, or with walls filled with clay and covered with planks on both sides. Inside the church, the walls were covered with light-colored clay and likely decorated with, for example, patterned fabrics. This possibility is supported by findings of lumps of melted bronze along the outline of the walls and fragments of copper applications.