The church in Siesiki , with its heavy, austere exterior, is an example of the architecture that was built in the provincial centres of the Grand Duchy of Lithuania. However, being built of stone in an area where wooden buildings predominated, it was a sign of the dominance not only of the Roman Catholic Church, but also of its founders, the Dowmont-Siesicki family from the Wiłkomierski district .

In the literature, the church of St Bartholomew (originally Saviour) is believed to have been founded by Stanislaus (c.1470-1561) and his younger brothers Michał (Melchior) and Abraham Siesicki of Bawola Głowa Coat of Arms in 1537, sons of Gabriel Dowmont-Siesicki and Katarzyna of Połubiński. The form of the church, however, indicates that it was built a century later, i.e. around 1630 or even later (the Lithuanian Heritage Register gives the date as 1662-1668, which should rather refer to the renovation and re-consecration after the destruction of the Deluge). Similarly, it is stated that the tower of the church was raised by one storey in the 18th century. The church was repaired in 1818-1819, 1846 and in 1958. Modest frescos were painted on the chancel vault in the 20th century. As late as in the 19th century, the square next to the temple was surrounded by a stone and brick wall.

The described temple in Siesiki is probably the third one. The first one existed before 1441 , when Bartholomew Snukis Urbanowicz granted the parish the Ponatery estate with the obligation to build a chapel there. The second one may have actually been built in 1537, the year referred to in the guidebooks . It was a temple that fell into the hands of Calvinists, as the Lithuanian nobility at that time was massively in favour of Protestant currents. The current temple was built by Kacper, son of Melchior , a town clerk and constable of the Vilnia County. It was the time of the return of the Lithuanian elite to the Roman Catholic Church.

An anonymous masonry workshop built a rectangular building as a three-nave hall with a three-sided enclosed presbytery with a sacristy and treasury and a massive four-storey tower in the façade to which two small staircases were attached. The lower storeys of the tower are quadrilateral and the last storey is octagonal. Its successive parts step upwards. The modest rhythmic decoration of the side façades consists of lisens enclosing semicircular windows. They support the cornice surrounding the building. The main entrance is located in the tower, on the axis of the building. The shape of the portal is interesting, as it embraces a profiled and transformed arch in the shape of a donkey's back. Above it, still in the mid-19th century, was the Domont-Siesickis' coat of arms.

The interior of the church is covered by cross vaults supported by massive pillars and servants at the side walls. Their capitals are simplified geometric forms. The vaults have wide mouldings arranged in the nave in picturesque leafy and quadrilateral forms. The church has a basement with extensive crypts.

The oldest part of the church furnishings are the two supper benches, which were probably made in the first third of the 17th century. The crucifix and numerous paintings from the 18th to 19th centuries should also be mentioned. Particularly interesting is the series of large canvases depicting the Four Evangelists, which was created in connection with the activities of the first Faculty of Fine Arts in the former Republic , which was established at Vilnius University in 1797 . Unfortunately, it is not known which of the students may have done this work. In addition, modest neo-Gothic altars have been preserved in the interior . In the tower, a rarity, three old bells from 1692, 1701 and the 18th century have been preserved. Mention should also be made of the modest epitaphs of Konstanty Kamieński (d. 1830), frontier court judge, and his wife Marianna, née Kossakowska (d. 1831), and above all of the epitaph of Konstantin Dowgiałło (d. 1790) "heir(s) to the Siesik estate" with the Zadora coat of arms. The shape of the latter is in keeping with the plaques made around 1800 by Vilnius stonemasons, so it may be suspected that it was commissioned from one of the capital's workshops.

The organ has a unique history . It was originally located in the Basilian Church in Barany (near Vilnius). The monks signed a contract for their construction in 1782 with the organ-builder Michael Zell. It was placed in Siesiki in 1847, which is documented by the inscription on the brass plate on the side of the organ. The instrument was extensively rebuilt at the beginning of the 20th century by the master Augustin Montwiłł. At present, the organ has two manuals and pedals. The organ prospectus, on the other hand, is a typical architectural form used by the masters of the Vilnius school of the late Baroque. It is a seven-element composition consisting of two high and polygonal towers that embrace a low, triangular tower, which are connected by curved cornices.