I don't have sources, but offhand I can discuss the architecture and development of Barcelona:

There's a few portions of the city, and they evolved in chunks.

There's the 'Old Town' (Ciutat Vella in Catalan), but it most commonly called the Gothic Quarter (Gòtic in Catalan), and this is the oldest portion of Barcelona.

If you've heard of the pedestrian street 'Las Ramblas', that road cuts right though the center of the Gothic Quarter.

Barcelona was originally a Roman (edit: I was wrong! Carthaginian!) city called Barcino and was surrounded by a wall. If you've been to Barcelona, it's got crazy streets (some are only shoulder-width!) and there doesn't seem to be any pattern to the streets.

In 1859, there was a planned expansion of the city. This was called "L'Eixample" (pronounced: eschamplah) and literally means "The Expansion". When you look at Barcelona from a helicopter, there's a repeating pattern of square blocks with their corners cut off. This picture shows the difference between the Eixample and the Old Town.

One of the principles that was instrumental in the planning of Eixample was the idea of 'Human-Scale Architecture'. This means that when someone is standing on the roof of a building and looking down, you can recognize their face from the street. Because of this, buildings were only built four or five stories tall (sidenote: in my experience, you never feel like you're in a big city. Compare that to NYC or Chicago, where it feels like a glass canyon.)

When you're outside the Eixample, Barcelona feels like any other big American city: skyscapers covered in glass, wide roads, Ikea's, few pedestrians, lots of traffic (personal note: feels much more uncomfortable than Eixample). I don't know specifically about the development of the outskirts, so I can't comment on that.

Regarding 'most famous building', I can't speak definitively to pre-Gaudí (Gaudí was about 7 years old when Eixample was started, so that portion of Barcelona's development was already underway when he moved there.), but I do know that current day Barcelona's most famous spots, like Sagrada Família and Parc Guell, which are what most people think of when Barcelona comes to mind, were made (or planned) by Gaudí.

Hope this helps!