An austere Romanesque building from the outside, the Lisbon Cathedral (Sé de Lisboa) has some lovely treasures inside. It dates from 1150 and was built this solidly to repel attacks from the Moors. It didn't do much to ward off earthquake damage in 1344 and 1755 and the cathedral we see today has been much repaired.
Inside you'll find the font where Saint Anthony of Padua is said to have been baptized in 1195 and a 14th century chapel by Bartholomeu Joanes. But its in the sacristy that the real treasures are found: relics, icons and 15th and 16th century religious art. The medieval cloister is also worth a look.
Located in the Alfama district, it's best to catch tram 28 to Graca. To see the Cloister and Sacristy there is a small cost and it closes for lunch from 1pm - 2pm.