Lisbon downtown (a baixa) was built at the end of the 18th century, after the almost complete destruction of the city by the 1755 earthquake. Although some hurts already exist, the architectural style is thus fairly homogeneous.
The urban mesh is geometrical (the streets cross at right angles), which is a common pattern in the USA for instance. The Bairro Alto quarter follows the same pattern but the houses were of a lower class. That contrasts with the patterns of Alfama and Mouraria quarters whose urban patterns (not the houses) date back to the Moors dominance of the city (from the 8th century up to 1147).