I found this history of Capella d'en Marcús: Well worth a peek is this diminutive 12th-century chapel nestling in the tiny Placeta d'en Marcus, near the junction of calles Montcada and Carders (the latter means "Woolcomber's Street"). Originally conceived by one Bernat Marcus as a sanctuary for luckless travelers who reached the city after the gates had been closed, the chapel may have been headquarters of the country's very first postal service.
I walk by this every day and it recently caught my eye, it's so beautiful.