One of Porto's most stunning buildings can be found just north of the Ribeira, Porto's waterfront, in the Infante D. Henrique Square. The Palácio da Bolsa or Bolsa Palace was never actually a palace for royals - it's the old stock exchange building of Porto where, to this day, economic business activity is conducted.
Originally, the land on which the palace was built was home to the St Francis Convent, founded in the 13th century. It was partially destroyed in 1832 during the Liberal Wars, after which time the ruins and land were given to the city by Queen Mary II in 1841 to build the Commercial Association. It was structurally finished in 1850, although the interior decorations took until 1910! As you walk through, you can see why it took so long to complete - it's seriously impressive.
The building is simply exquisite. Having visited numerous architecturally beautifully and well-designer buildings over the years, I've found it harder and harder to be blown away by places as I start to rack them up - but this one managed to do just that. Each room was more ornate, intricate and over the top than the one before. I suggest trying to come at the end of the day so you can get photos without all the others!
How to Visit
You can buy tickets online or in person at the ticket office when you arrive. Tours are offered in Portuguese, Spanish, French and English, however there are no set times for each language per day; rather, they are offered on a first come first serve basis based on who shows up that day asking for whichever language. In principle this sounds a bit bizarre as you may show up and not have a tour in your language available, but I showed up at 5:45pm and they told me they were running a tour in English at 6:30pm (the last tour of the day). Maybe I was lucky, but I also think that so long as there's a handful of people wanting to go in your language, they'll run one - my tour only had five people on it.
Note that you can only visit with a guided tour as it is still a functioning office building so you cannot wander around at leisure.