The modern look of St. Cyril’s Church, in the forms of Ukrainian Baroque, only conditionally resembles its original architecture. While the interior of the temple has hardly changed since the 12th century, the church, built by local masters, reflected a new direction of ancient Rus’ architecture, which developed in the late 11th – first third of the 12th century in Chernihiv architecture. In addition to Byzantine and Kyiv architecture, Romanesque architecture had a significant influence on the formation of this new construction school. After the establishment of the Chernihiv princely dynasty of Olhovychi in Kyiv in the 1140s and the arrival (of Kyiv) of the Chernihiv artel, this trend spread to almost all ancient Rus’ lands.
St. Cyril’s Church belongs to the type of cross-domed, six-column, three-nave church with one cupola and arched gable (zakomar) covering. On the western, northern and southern sides of the church small outbuildings adjoined it, the foundations of which were discovered during archaeological research. The three portals of the church had a three-tiered profile and ended with semicircular arches (architectural forms and fragments of the original fresco of the southern portal have been preserved). The hemispherical dome is located on a high, round in plan, drum mounted on a powerful quadrangle. The facades of the church were divided by pilasters in accordance with the internal division of the building. The arches of the arched gables rested on the pilasters. Under the arched gables and above the cornice of the dome was an arched frieze (its fragments are preserved on the southern facade). The apses and drum of the dome were decorated with half-columns.
The church is a rectangle elongated along the axis “west-east” in plan, its dimensions – 31 x 18.4 m. The church is divided by three pillars into three longitudinal (naves) and three transverse corridors. Longitudinal naves end in the east with altar apses. The central nave and the main transverse corridor (transept) form a spatial cross at the intersection. There is a dome above the center of the cross. The size of the domed space is 7 x 7.7 m. Between the western pair of pillars there is a wall with a wide arch leading from the nave to the narthex. The central part of the narthex is covered with a box vault, the lateral parts are covered with cross vaults. Archaeological excavations have revealed that in the southern part of the narthex there was a baptistery with a small apse, only fragments of which have survived. There are four arches in the walls of the narthex. The choirs have a narrow passage in the northern wall, the thickness of which is 1.98 m. The choirs are located above the narthex and the western centers of the side naves. In the southern ledge of the choirs is the prince’s prayer room, which looks like a small isolated room with an apse. The pilasters of the pillars on the choirs are decorated with half-columns. In the central part of the church the choirs open with arches. There is a passage in the wall of the southern apse. Its stairs led to a small room with an arched opening open towards the altar.
The church was built in the technique of even-layer masonry (“opus isodus”) with plinths measuring 30 x 26 x 4.5 cm; 32 x 22 x 5 cm. The ancient masonry (the probe is open on the western and southern facades) has been preserved around the perimeter of the walls to the level of the heels, as well as in the apses, pillars, supporting arches and 2/3 of the dome drum. Clay jugs (vowels) are used in the sails of the cross vaults, which serve to improve the acoustics. Local pyrophyllite slate is widely used in the church décor, the so-called red slate: ancient cornices on pillars and slabs in the north wall have preserved. The floor of the church was laid with smooth slate slabs, in the central part of the church they were inlaid with colored smalt. The ancient floor has not been preserved due to numerous repairs. In the 1960s, the floor level was leveled to the initial in order to restore the original architectural volume of the church.
The first evidence of the renovation of the church dates back to the beginning of the 17th century. Then the vault in the central part of the temple was moved, the facades were plastered. In the late 17th – early 18th centuries the gabled ceiling of the church was replaced by quadrilateral one, four side domes were installed on octagonal drums, the western façade was strengthened with two pylons, and an entablature was erected instead of arched gable covering.
As a result of the reconstruction of 1748-1760, the roof of the church became gabled, the domes received a faceted pear-shaped form, and a gable of a soft wavy outline was erected over the western facade. Numerous and various details of a stucco decor: windows and niches are decorated with flower garlands, plant shoots, acanthus leaves; portals – columns, capitals of the Ionic and Corinthian orders gave the architecture a special solemnity. The lush and expressive features of the Ukrainian Baroque still define the modern appearance of the monument. Scientific research of the architecture of St. Cyril’s Church was carried out by: A. Prakhov (1880-1882), I. Morgilevsky (1924-1928), Y. Aseev (1937-1939, 1944-1945), N. Kholostenko. 1955 – 1960).