The south facade was supposed to serve as a grand ceremonial entrance overlooking the lagoon until it was walled in 1503. Despite the Doge's Palace blocking a large portion of the view from the Grand Canal, its facade is nonetheless impressive. Artistic touches like sculptures and mosaics are present inside St. Mark's Basilica, and decorate it magically. Observe the Byzantine mosaic of the Virgin from the 13th century that was installed between the upper floor's arches, and don't miss the two gryphons in the first arch. Two marble pilasters (Pilastri Acritani) flank the entrance, adorned with exquisite sixth-century reliefs. The Porta dei Fiori, also known as the Door of Flowers, can be found on the building's north facade, facing the Piazzetta dei Leoncini. It features a stunning relief of the Nativity from the 13th century, surrounded by angels, prophets, and flourishing plants.

The narthex (entrance porch) of St. Mark's, like those of all Byzantine basilicas, serves as a sort of foyer before stepping inside the main church itself. Except for a St. Mark mosaic added to the vaulting in 1545, all of the mosaics in its domes and arches date from the 13th century. From right to left, they depict the Creation, the murder of Abel by Cain, the building of the Ark by Noah, the confusion at Babel, and the patriarchs Abraham, Joseph, and Moses. All three of the church's entrances are framed by carved marble columns with elaborate capitals dating back to the sixth to ninth centuries. A gallery overlooking Piazza San Marco inside St. Mark's Basilica, may be found above the narthex, where the four bronze horses were displayed until 1981.

The mosaics inside St. Mark's Basilica are the first thing you'll see, and they'll steal your breath away, but it's nearly impossible to pick a single highlight from this treasure-filled basilica. When taken along with the church's overall design, they create an appearance of Byzantine architecture, despite the interior's eclectic blend of classical and nineteenth-century styles. Byzantine doctrine divides a church's structure and purpose into earthly and heavenly parts; the dome over St. Mark's, made of small coloured glass pieces and gold leaf, is a shining example of this principle. The five domes, supported by massive pillars, measure about 13 metres in diameter and have 16 skylights. Basilica d'Oro, or Golden Basilica, gets its common name from the lavish gold mosaics that adorn the domes, a sum of 4,240 square metres.

It is easy to see why the golden retable in San Marco is regarded as one of the finest examples of Byzantine ecclesiastical art. At 3.45 metres in length and 1.45 metres in height, it is undoubtedly one of Europe's most impressive altarpieces. The round gold and enamel plates around the edge are the earliest parts. The artwork is a combination of the work of Venetian and Constantinople artisans and mixes elaborate goldwork with enamelled medallions showing the lives of Christ, St. Mark, and other biblical subjects. Over the course of 500 years, various features were added inside St. Mark's Basilica.

Following the principles of Byzantine church design, St. Mark's crafted a floor that emphasises earthly solidarity to complement the domes and ceiling's depiction of the heavenly realm. Over 2,099 square metres of marble inlay are covered in elaborate geometrical and natural designs, all in warm earth tones. Even though the floor is predominantly geometric, it is brightened with elaborate animal and floral motifs carved from small bits of marble or even glass. It is speculated that the floor's creators, like the mosaicists who worked above them, were Greek or Constantinople natives.

Chapels honouring the Virgin Mary and other saints are housed inside St. Mark's Basilica ‘s short arms, which extend from each side of the main dome. The Gothic altar sculptures and 15th-century mosaic vaulting in Cappella della Madonna dei Máscoli. The remains of Sant'Isidoro are housed in a wall tomb in the chapel, which is also ornamented with mosaics from the middle of the 14th century. Cappella della Madonna Nicopeia is located on the north side of the transept, and its altar is where the magnificent Byzantine image of the Madonna Nicopeia may be seen. Formerly the Doge's chapel, Cappella di San Clemente can be found in the south transept and features a wall of columns from the 14th century, as well as a mosaic depicting St. Clement from the 12th century, in the apse.

An ornate rood-screen separates the choir from the nave. From 1394 to 1404, Jacopello and Pierpaolo dalle Masegne created the silver Crucifix and figures of the Madonna, St. Mark, and the Apostles that adorn the lectern. Traditional public introductions of the newly elected Doge took place from the right-hand pulpit. Jacopo Sansovino spent 1537–1541 casting bronze reliefs depicting episodes from the life of St. Mark, whose remains are kept at the High Altar. Three hundred and twenty-four reliefs depicting events from the lives of Jesus and Mary adorn the four columns that support the ciborium, the architectural canopy just above the altar.

The Venetians returned with a treasure trove of artefacts from Constantinople in 1204. These can be viewed in the Treasury, which is accessible from the south transept's corner. Even after Napoleon's looting and the selling of certain gems to pay for necessary renovation of the cathedral in the early 1800s, the church's treasury still contains 283 pieces in gold, silver, and other valuable materials, making it one of the most important and richest in Europe. One hundred and ten Byzantine reliquaries, made between the 11th and 13th centuries and adorned with valuable stones, were found among the loot from Constantinople. Later additions from popes, European lords, or doges supplement the collections with donations of gold and silver Byzantine icons, glassware, chalices, Venetian filigree, enamelled goldwork, Islamic art, and more.

A little door near the church's main entrance leads to the Marciano Museum, where the originals of the four iconic horses that originally stood on the gallery above the main entrance are on display. These gilded copper horses have seen quite the world: they were part of the war booty stolen from Constantinople, then robbed again by Napoleon, then returned to the gallery, and were ultimately transferred here for preservation. Other highlights include a gorgeous cover for the Pala d'Oro created in 1345 by Paolo Veneziano and his sons depicting scenes from the life of St. Mark, and excellent Gobelin tapestries from the 13th to 16th centuries.