Looking like it was beamed in from Spain, this handsome 1844 cathedral was designed by the famed architect Carlo Rossi, who left his mark on the neoclassical shape of St Petersburg. Built where the refectory of its neighbour St Catherine's once stood, it's still one of Tallinn’s only Catholic churches, largely serving the Polish and Lithuanian communities (although there's also an English-language mass at 6pm each Saturday). The front courtyard offers some respite from the summertime bustle.
When St Catherine's next door is closed, you may be able to enter its cloister from a door adjoining the cathedral's entrance, for €2.