The building in which Café Engel is located is one of the oldest in Helsinki. The ground floor was built in 1765 in the place of an old pipe factory. The architect is unknown. In the 1830´s two floors were added and Carl Ludvig Engel (1778-1840) designed the facade.
Engel begun his studies in his hometown Berlin and continued them later in St. Petersburg during the reign of Alexander the II. He designed many Empire style buildings around the Senate Square including The Dome of Helsinki, The University Hall and The University Library.

More information:
Wikipedia article: Carl Ludvig Engel