The town is famed for its "Old Town" (Gamla Stan in Swedish), a dense mediaeval street pattern with predominantly wooden houses. The Old Town came close to being demolished in the 19th century by a new urban plan for the city. The plan was cancelled due to a popular resistance headed by Count Louis Sparre
The central point of the old town is the medieval, stone and brick Porvoo Cathedral which gave its name to the Porvoo Communion – an inter-church agreement between a number of Anglican and Lutheran denominations. The cathedral was damaged by fire on 29 May 2006: the roof was totally destroyed but the interior is largely intact. A drunken youth had played with fire at the church, unaware of recent tarwork and nearby tar containers, accidentally causing a large fire. He was later sentenced to a short prison term and restitutions of 4.3 million Euros.
The red-colored wooden storage buildings on the riverside are a proposed UNESCO world heritage site. Already by the early 19th century the authorities understood the value of the old town, and so with the need for growth a plan was made for a 'new town' built adjacent to the old town, following a grid plan but with houses also built in wood.
By the end of the 20th century there was pressure to develop the essentially untouched western side of the river. There was concern that growth would necessitate the construction of a second bridge across the river into the town, thus putting further strain on the wooden town. An architectural competition was held in 1990, the winning entry of which proposed building the second bridge. Plans for the western side of the river have progressed under the direction of architect Tuomas Siitonen, and both a vehicle bridge and a pedestrian bridge have been built. The design for new housing is based on a typology derived from the old store houses on the opposite side of the river. Yet another new development entails the construction of a large business park called
