The Royal, or Wołłowicz, Chapel was built in the late 15th century by the King of Poland and Grand Duke of Lithuania Casimir Jagiellon. The crypt of this chapel became the burial place for members of the royal family, including the famously pious Prince Casimir.

When the Church allowed the veneration of St Casimir, Sigismund III Vasa, the ruler of Lithuania and Poland, began to build a new Chapel of St Casimir in the southeast corner, on the site of the former chapel maintained by the Bishop of Vilnius Eustachius Wołłowicz (1572–1630). The two chapels swapped places, and the Royal Chapel became the Wołłowicz Chapel. The chapel was built and decorated by Eustachius Wołłowicz before 1630.

The altarpiece of the Cathedral Basilica’s main altar has three parts. The first, somewhat higher up than the others, bears a symbol of Divine Providence, above which there rises a cross with the figure of Christ. The second part features an elaborate tabernacle.

A tabernacle is an ornate box meant to hold the Most Blessed Sacrament. The Cathedral tabernacle was decorated with the scenes portraying the events of Holy Thursday: the Last Supper, Jesus washing the feet of his disciples, and his prayer in the Garden of Olives. Above the tabernacle there is a domed niche for a cross or a monstrance, beside which there are the statues of St. Casimir and St. Stanislaus – the patron saints of Lithuania and the diocese of Vilnius.

The new altar, where Holy Mass is now celebrated, was installed in 2007 in line with the indications of Vatican Council II.

The paintings of the Twelve Apostles on the pillars of the central nave are by Franciszek Smuglewicz.

The artist Costantino Villani was invited from Italy to create 16 large paintings specifically for the cathedral. It is the most spectacular series of paintings on themes from Holy Scripture in Lithuania.

Facing the altar, scenes from the New Testament are depicted on the left side, and scenes from the Old Testament are depicted on the right.

On the right side, eight paintings representing stories from the Old Testament hang on the walls above the chapels between the pilasters. Walking from St Casimir’s Chapel towards the organ loft, they are: The Creation, Cain and Abel, Abraham’s Sacrifice of Isaac, Jacob’s Dream, Joseph in Prison, Moses at Mount Sinai, Joshua’s Victory and The Judgement of Solomon.

There are another eight paintings by Costantino Villani on the left side. Walking from the sacristy towards the organ loft, they are: The Annunciation by the Archangel Gabriel, The Nativity of Christ, The Massacre of the Innocents, The Adoration of the Magi, The Flight into Egypt, The Baptism of Christ, Jesus is Laid in the Tomb and The Descent of the Holy Spirit.

The highlight of the chapel is the painting "Our Lady of St Michael", which is more usually called "The Madonna of the Sapiehas". This image of the Blessed Virgin Mary with the Franciscan saints Francis and Bernardine has been famous for its miracles since the 16th century. Formerly kept in the Bernardine Church, the painting later belonged to the Bernardine convent’s Church of St Michael the Archangel. The name "The Madonna of the Sapiehas" refers to the name of the family who endowed the convent. Miracles worked in the presence of the painting were recorded, and on 8 September 1750 the image was decorated with crowns sent by Pope Benedict XIV (the frame and the crowns have not survived). It was the fourth holy image in the Grand Duchy of Lithuania to be crowned (and the second oldest in Lithuania today).

In 2020, through the efforts of the Cathedral community, the crowns of Our Lady and Infant Jesus were restored and adorn the painting again.

The decoration of the chapel’s dome refers to Casimir’s piety and virtue. Sculptures symbolising temperance, justice, fortitude and prudence stand in the niches between the windows.

The virtue of Justice is represented by a woman holding a pair of scales, a symbol of fairness and impartiality, in her right hand. In her left hand, she holds a sword to protect the righteous and punish the guilty. Next to her is an ostrich, a symbol of justice. It was believed that this bird can digest iron, and all its feathers are of equal length.

The virtue of Fortitude is symbolised by an armour-clad woman. In her right hand, she is holding an oak staff, showing strength of spirit; and with her left hand she is leaning on a column, signifying constancy. Next to her is a lion, a symbol of wisdom and power.

The figure depicting the virtue of Prudence is holding a mirror, a symbol of self-knowledge. In her right hand she is holding a fish, referring to well-judged, prudent actions. Next to her is an elk, which was considered a wise animal, for even when running swiftly it does not get caught up in shrubbery.

The figure representing the virtue of Temperance is diluting wine with water. An elephant standing nearby also embodies moderation. It was believed that this animal never eats more than necessary, and puts aside the rest for the next day.

There are four painted compositions in the dome’s pendentives: Chastity and Virtue above the entrance (by Franciszek Smuglewicz), and St Casimir’s Hymn to the Blessed Virgin Mary and St Casimir Accompanied to Heaven above the altar (by Michelangelo Palloni). At the apex of the dome is a dove, a symbol of the Holy Spirit.

The Chapel of St Casimir is a particularly holy site in Vilnius Cathedral, and a special chapel that continues to attract tourists and pilgrims. Casimir (1458–1484) was the second son of Grand Duke Casimir of Lithuania (ruled 1440–1492). He achieved fame both for his ruling skills and for his pious way of life. He died very young, and was buried in Vilnius Cathedral, where believers soon started to witness various miracles. The canonisation process started not long after his death, but it remained unfinished throughout the 16th century. The prince was officially acknowledged as a saint in the early 17th century, a fact celebrated with huge festivities in Vilnius in 1604.

The present chapel was built in 1623–1636, at the initiative of Sigismund III Vasa, the grandson of St Casimir’s brother, and the great grandson Ladislaus IV Vasa. It was designed and built by the Italian architects Matteo Castello and Costante Tencalla. The chapel was the first building in the Grand Duchy of Lithuania to be decorated mainly with marble. Its exterior was embellished with slabs of sandstone, which were rare here. The prince’s remains were transferred to the chapel in 1636. In the late 17th century, the dome and the altar were adorned with magnificent works of stucco.

Albertus Goštautas (1480–1539) was the son of Martinus Goštautas and Anne, the duchess of Alšėnai. He became chancellor of the Grand Duchy of Lithuania and voivode of Vilnius, and was called ‘the Viceroy of Lithuania’ due to his political influence in the first half of the 16th century. He paid much attention to the chapel, and was one of the first well-educated Lithuanian nobles to make impressive endowments to commemorate his family. The extravagant red marble monument erected after Albertus Goštautas’ death was most likely commissioned by his son Stanislaus, the first husband of Queen Barbara Radvilaitė (Radziwiłł). The Goštautas line ended with his death, and the monument remained unfinished: only a tombstone plaque has survived, with a relief image of the noble in Renaissance armour holding a sword and a banner.

The plaque was partly destroyed during the wars of the mid-17th century, but it still preserves the features of Albertus Goštautas, one of Lithuania’s most prominent nobles, and his coat of arms, which combines the crests of his father, his mother, and both grandparents. The surviving tombstone plaque of Albertus Goštautas in relief, with features of Gothic and Renaissance art, is the earliest work of memorial sculpture in Lithuania.