The Split Synagogue in Croatia is one of the oldest European synagogues still in its original use.

In the 16th century, Sephardic Jews arrived from the Iberian Peninsula and created the new structure by converting the second floor of two attached medieval houses.
The current appearance of the interior was formed around 1728. The most sacred part of the place of worship, the Aron Hakodesh, facing Jerusalem and made in the Classicist style of black and white marble, is built into the western wall of Diocletian’s Palace.

Source: Adam Esses