So, this is the Cathedral, or with its full name the “Basilica cattedrale di San Giusto Martire“. Seriously mixed styles, huh?
That’s something you see frequently in Italy. Churches started as re-purposed Roman temples or secular buildings. When the first church in this place was built, in the 6th century, the Western Empire had already fallen. Trieste was a small city at the fringes of the Eastern Empire, frequently besieged, sometimes conquered. Every time those churches were destroyed and rebuilt, fragments were reused. The current building is from the 14th century, when two older churches were fused and the Gothic rose window was broken into the wall as a tribute to the then modern architectural style.
One could see this fusion of styles as impure, but for me it contributes much to the feeling of age, history and individuality.