Theatro Victória is a handsome survival from the period when Amazon river towns built civic monuments to match their commercial ambitions. Its facade and auditorium belong to the same broad rubber-era world as the grand theatres of Manaus and Belém, though on a smaller Santarém scale.
Even if you only see it from outside, it adds a cultural stop to a city many travellers treat mainly as the jump-off for Alter do Chão. Check locally for performances or access, because the building is most interesting when you can connect the architecture with actual city life.