'In Summo' is a Latin phrase referring to this church's position 'on high' or 'on the heights' of Ostrów Tumski. In fact, this small Gothic church was built in the early-15th century on the exact position of Mieszko I's 10th-century palace, even using some of the same foundations. A unique glass installation surrounding the building traces the outline of that palace - Ostrów Tumski's first royal residence and Christian temple.

As for the church itself, it is modelled on the West Pomeranian building style, with a three-nave hall, star vaulting and polychromatic decorations; the altar was designed by Poznań artist Wacław Taranczewski in 1954. The tall facade with its abrupt edge almost make the church appear as if it is unfinished, and distinctive grooves in the eastern wall have lead to speculation that it was used to sharpen swords. On the brink of being torn down in the 19th century, the church was instead used as a hospital and then granary, and has since been lovingly restored.