The Basilica di San Marco is the most Byzantine building west of Istanbul — 5 domes, 8,500m² of golden mosaics, a jeweled altarpiece (Pala d'Oro) with 1,927 gemstones, and 4 bronze horses looted from Constantinople during the Fourth Crusade (1204) that Venice has displayed on the facade for 800 years despite repeated requests to return them. Entry is free. The line is 45-90 minutes. The €3 skip-the-line reservation is the best €3 you'll spend in Venice. Venice museums →
The mosaics (look UP everywhere): 8,500m² of gold-tessera mosaics (12th-16th century) covering every dome, arch, and vault. The Genesis Dome (first dome from entrance — Creation scenes, 13th century). The Ascension Dome (central — Christ ascending surrounded by angels). The Pentecost Dome (first dome, tongues of flame descending on the Apostles). The tip: bring small binoculars or zoom on your phone — the dome mosaics are 25m above you and the detail rewards close looking.
Pala d'Oro (€5 supplement, behind the high altar): A golden altarpiece (3.48m × 1.40m) encrusted with 1,927 gemstones — rubies, sapphires, emeralds, pearls, garnets, amethysts — assembled over 5 centuries (10th-14th). The most valuable piece of goldsmith work in Europe. Bronze Horses (Museo di San Marco, €7): The ORIGINAL 4 horses are inside the museum (the ones on the facade are copies since 1982). Greek or Roman bronze (2nd-4th century AD), looted from Constantinople's Hippodrome in 1204. Napoleon looted them to Paris in 1797. Venice got them back in 1815. Constantinople never did. The terrace: The €7 museum ticket includes access to the loggia — the best close-up view of Piazza San Marco from above.
Piazza San Marco. Free entry (Pala d'Oro €5 extra, Museum/Horses/Terrace €7 extra). Skip-the-line: book on venetoinside.com (€3 reservation) — saves 45-90 minutes. Dress code ENFORCED: Shoulders covered, knees covered, no large bags (free bag storage at Ateneo San Basso nearby). No photos inside (enforced by guards). Best time: 9:30am opening (before tours arrive). Acqua alta: The basilica floods during high water events — raised walkways are installed, but the experience of seeing the golden floor under 10cm of water is surreal.