St. Mark's plan is Greek cross. Both arms of the cross are subdivided
into nave and two aisles.
Beyond the transept, delimited by the iconostasis, the area of the eastern
arm is occupied by the presbytery in the centre and, at the sides, by
the chapels of St. Peter to the north and St. Clement to the south.
At the bottom of the presbytery, abutting the apse, there is an altar
on a platform once raised by five steps, for deposition of the Most Holy.
The transept extremities close with a rectilinear wall. To the north they
take in the walls of the St. Isadore and Mascoli chapels, and to the south
those of access to the Ducal Palace. On the west and north sides the church
is surrounded by a narthex in which, at the southern end, there was the
"sea gate", now occupied by the chapel of cardinal Giovambattista Zen.
The main entrance is from the West facade. To right and left are the
St. Clement and St. Peter entrances. At the northern end of the façade,
the St. Alipius entrance. In the northern arm the Door of Flowers is also
closed with a bronze gate.
The church is accessed from the narthex by means of four doors: the central
one, the St. Clement and the St. Peter, in correspondence to the chapels
of the same name and, to the north the Door of the Virgin or of St. John.
The baptistery, built on the southern frontage at the limit of the sea
gate, between the gate and an ancient corner-tower, features two cupolas
and a vault connecting it to the structures of the Zen chapel. The tower,
of uncertain function and transformed with the creation of the third St.
Mark's, is connected internally to the church and to the walls of the
building incorporated into the head of the south transept. Today it houses
the Treasure and the Sanctuary with the relics.
Access to the sacristy, enlarged at the end of the 15th century, is from
the presbytery and St. Peter's chapel. Adjacent to the sacristy there
is the 15th century church of St. Theodore.
The nave and two aisle crypt with apse is beneath the presbytery and
the side chapels. In the nave, beneath the high altar, there is the ancient
chapel where the evangelist's remains were kept. The crypt has an intersecting
barrel-vault ceiling supported by small columns. To the west of the crypt,
an area known as the "retro-crypt " contains the tombs of all the patriarchs
of Venice since 1807.
Two areas of the church may be defined: the ducal area in the south transept,
closely connected with the palace by passages and windows at various levels,
and the St. Mark's primicerius' and priests' area in the north transept,
linked to their respective lodgings.
The height and size of the buildings around the church reduced the amount
of light reaching the latter, so at the beginning of the 15th century
the Serenissima decided to create two great openings: the window of the
horses on the façade and the rosette in the south transept overlooking
the doge's palace.
The cupolas - the Ascension in the centre, the Prophets over the presbytery,
the Pentecost over the nave, the St. John over the north arm and the St.
Leonard over the south arms of the transept consist of half-spheres in
masonry standing on great support vaults covered by wooden ones of larger
size topped with a small cupola bearing a gilded cosmic cross.