The beginnings of the parish church in Krosno are connected with the location of the city under the Magdeburg Law, which took place in the 40’s of the 14th century. Church was merged directly into the city walls and became an important element of the medieval fortifications of the city. Until today, a readable element of the medieval fortifications, is the ground floor of the 14th-century defense tower, preserved in the south-west corner of the nave of the church. Originally, it was a high tower serving as a municipal watchtower and church belfry.
In the fifteenth century, when Krosno experienced significant development and enrichment, the parish church was also expanded. In the first half of this century, from the north, chapels of St. Anna (before 1402) and St. Adalbert (before 1448), and from the southern side the Society of the Blessed Virgin Mary (before 1446) and St. Peter and Paul (before 1512) were erected. At the end of the fifteenth century, the presbytery was added and closed with a new, late gothic vault. The turn of the fifteenth and sixteenth centuries brought the enlargement of the sacristy to the east and its superstructure for the treasury room on the first floor.
In 1638 a dangerous fire occurred in Krosno, but in the light of newer research, the church did not suffer much damages. Early modern transformations of the temple are the result of changes introduced by the councilor Wojciech Robert Portius in the 17th century. Work carried out under the supervision of Italian murator Wincent Petroni, transformed the interior of the temple from gothic to mannerist. From this time comes the barrel vault in the nave, supported by buttresses introduced into the interior of the temple. At that time, Petroni rebuilt the chapel of St. Peter and Paul into the dome mausoleum of the Portius family. In this form, with only minor changes, the Krosno parish church survived until the beginning of the 20th century, when many previously lost features were restored and unveiled.
The church consists of a three-span, three-side ended chancel with the sacristy from the north and a five-span nave. Several chapels are added to the nave, from the north they are chapels of St. Adalbert, St. John of Nepomuk and St. Anna, from the south in the ground floor of the former tower are added porch and then the chapels of the Mother of God and Peter and Paul. Outside, the chancel is buttressed, between buttresses there are pointed gothic windows. The western façade is crowned with a triangular gable with steps on the sides and fragmented with five pointed blendes. In its lower part there is a large pointed window with tracery. Inside, the chancel is covered with a late gothic, net vault.
Website zabytkowekoscioly.net, Krosno, kościół Trójcy Świętej.