The abbey was founded in the second half of the 11th century by Casimir the Restorer or by Bolesław the Bold for monks brought from Rhineland and Bavaria. The Benedictines were then settled in a fortified settlement, which dated back to the around 8th – 10th century. In 1065, Bolesław endowed the abbey with large grants, the income of which allowed for the construction of the church and abbey buildings. The monastic church was first recorded in 1179, when Pope Alexander III approved the burial of lay people within the abbey.
One of the main tasks of the abbey, located on the outskirts of Greater Poland and Kujawy regions, was in the initial period of its existence to conduct a missionary action in Mazovia and Pomerania. The abbey of St. Adalbert in Płock was most likely a trace of the presence of the Benedictines from Mogilno in these areas, the monastery also had the church of St. John the Baptist and St. John the Evangelist in Kołobrzeg. Political activity and supporting the Church organization of the Gniezno diocese were no less important, which at the beginning of the 13th century involved the convent in a conflict between Archbishop Henry Kietlicz and the prince Władysław Laskonogi, and consequently ended in almost two years of exile of the Benedictines.
In the thirteenth century, the abbey was destroyed as a result of the invasion of the troops of prince Władysław Odonic in 1225 or 1230. The reconstruction was carried out in the late Romanesque style, and subsequent works from the fifteenth and sixteenth centuries gave the abbey Gothic features. The reconstruction after the fire of 1499 was especially primed, after which the transept with side chapels was removed, and late-Gothic vaults were installed in the nave and chancel. In the 16th century, the abbey was to be fortified (fortalicio monasteri) and adapted to serve as the seat of the Gniezno suffragen bishops, who were also abbots. From the second half of the 17th century, their role was taken by the commendatory abbots or their proxies, which contributed to the decline of the political role of the monastery and, as a consequence, to the deterioration of its buildings. It was not until the 18th century that the Benedictine community began to flourish again and rebuilt the monastery complex in the Baroque style. These works were carried out in the years 1760-1797.
In 1773, after the first partition of Poland, Mogilno was under Prussian rule, the authorities of which forbade joining the novitiate. In 1833 the abbey was completely closed, and the buildings were given to local Catholics and Protestants for a school. After 1880, the monastery housed a hospital, although the church still served as a parish. In 1934 the monument was renovated, and during World War II, the German Nazis set up there a prisoner-of-war camp, a prison for Jewish women and a hospital. In the 1970s, archaeological and historical researches were undertaken in the abbey, and the monument was renovated.
The abbey was situated on a peninsula that cut into the waters of Lake Mogileńskie. In the Middle Ages, this place was surrounded by water and swamps, which created favorable defensive conditions. A unique feature was the location of the monastery within the settlement. Originally, the abbey occupied the southern part of the developing town, surrounded on three sides by lake and wetlands. The convenient location of the settlement was used, and defense was increased over time by digging a ditch to create an artificial island.
Abbey consisted of a church, enclosure buildings and probably smaller auxiliary and farm buildings. The monastery church was built in the Romanesque part of granite. Originally it was a three-bay basilica with two aisles, a transept and a four-sided chancel closed in the east with an apse of a radius 2.3 meters. The west façade was crowned with a square tower with sides length of 5.6 meters, flanked by annexes being an extension of the aisles. In addition, chapels closed with apses were attached to the eastern walls of the transept. Their outer walls passed without a step, into the gable walls of the transept. The width of the central nave was 5.6 meters, the aisles were 2.9 meters wide, the transept was 2.7 / 2.9 x 9.8 meters, and the chancel was 7.3 x 8.2 meters inside.
There was no entrance portal in the western façade of the church, probably due to the proximity of the lake, so the spacious ground floor of the square tower could be used as a second, western choir. A complex of two Romanesque crypts was situated under the church: one under the chacnel and the other under the west tower. The eastern crypt was a single-space room, illuminated by three windows. In its western wall there were three semicircular niches, and in the side walls openings, connected with the later system of passages connecting both crypts. The entrance to the eastern crypt was provided by two flights of stairs embedded on the sides of the chancel arcade. It could have been covered with a high barrel vault. The western crypt was accessible by narrow stairs from the southern aisle. It was covered over four bays with a cross vault supported by arch bands, by a central pillar and wall pilasters ended with moulded impost cornices.
The division into the aisles of the Romanesque church was provided by three pairs of four-sided pillars, supporting the timber ceiling. They were not arranged symmetrically, because the western pair was only 1 meter from the tower, which in turn could indicate the functioning of the gallery there. In the Romanesque period, only the chancel was covered with the vault, where granite corbels were used. The entrances to the nave were located in the western parts of the northern and southern aisles.
In the first half of the thirteenth century, the chancel of the church was raised with the use of bricks laid in a monk bond, the nave was also raised a little later, also using bricks, but laid in the Flemish bond. In the chancel, the external facades were decorated with an arcaded frieze, and inside, the old vault was removed, in place of which, after raising, a flat ceiling or an open roof truss was installed. At the same time, the level of the chancel floor, originally about 2.5 meters higher, was lowered, which entailed the demolition of the Romanesque vault of the eastern crypt.
As a result of another, late-Gothic reconstruction from the beginning of the 16th century, the former flat ceilings were replaced with an eight-pointed stellar vault in the central nave and a diamond vault in the aisles. The central nave was supported by brick, octagonal pillars with pilaster strips, which supported the ribs of vault. The transept, demolished along with the side chapels, no longer functioned at that time, but two old cross-shaped pillars of the transept at the chancel arcade were preserved, so the new nave had to be, like the old one, slightly narrower than the chancel.
The monastery buildings were probably of timber construction until the 13th century. Erected gradually in the Gothic period, they finally obtained the form of three brick wings attached to the church from the south. The whole was originally connected by a gallery (cloister), forming a closed quadrangle with a free garden in the middle. In it, in the eleventh century, a Romanesque well was laid, filled in around the 15th century, with an original depth of about 14 meters. The southern wing housed the abbot’s apartment, refectory and monastery kitchen; in the east wing there was a dormitory on the first floor and a chapter house in the ground floor, and finally the west wing was occupied by auxiliary, utility and lay brothers rooms.
The entire monastery complex, from the 13th or 14th century, was surrounded by a 1.2 – 1.3 meters thick stone wall, reinforced from the side of the bridge by a 4.2 x 5 meter four-sided tower, fastened with buttresses on the south side. In the 15th century, a belfry was built of bricks, and buildings were added to the walls from the inside during the Gothic period. The gate to the abbey was located next to the western corner of the abbey and separated the abbots part of the complex from the rooms occupied by the convent. A drawbridge was to lead to it. Above the gate passage, about 10 meters long, there was a tower measuring 2.4 x 3.4 meters, reinforced with two buttresses.
Numerous rebuildings obliterated the original spatial layout and the shape of the monastery church, with the exception of the western crypt, which is considered to be one of the oldest surviving vaulted interiors in Poland. The eastern crypt has also survived, but its vaylt is not primary, as it covers two side windows. The walls of the transept and its apses have not survived, except for the foundations and the lowest parts of walls. Of the three pairs of Romanesque pillars, only one is visible, the eastern one, covered at a later date, while the rest are late Gothic. The west tower was mostly demolished in the 18th century, in the place of which two Baroque ones were erected, and the windows of the nave, chancel and central apse were also transformed in this style. The buildings of the monastic enclosure lost all of the medieval stylistic features.
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