The cathedral on the San Giusto hill is the most important church in Trieste — and one of the most unusual in Italy. It is actually two separate Romanesque basilicas that were merged into one in the 14th century, giving it an asymmetrical facade and a peculiar dual-nave interior that rewards close inspection. The campanile incorporates fragments of a Roman temple, and Roman ruins are visible in the square outside.
The interior is subdued and ancient. The Byzantine mosaics in the apses — Christ flanked by apostles, the Virgin enthroned — date from the 12th and 13th centuries and have survived the city's various occupations intact. The main square in front of the cathedral sits at the top of the Capitoline Hill, where there was once a Roman forum.
The hill is about a 15-minute walk from Piazza Unità d'Italia, or you can take the steps from the old city. Combine with a visit to the adjacent Castello di San Giusto for the views.