>> I highly recommend the guided tour if you're ever in Barcelona. The history of it's engineering and construction is fascinating even if you're not that into Gaudi, or cathedrals.

I can absolutely agree on this. As a staunch atheist I have no relationship to religion or the catholic church, but the creativity and dedication of this man is really fascinating -- even his death had a relation to the building. The architecture is truly awe inspiring and so rich in detail, you could surely spend several days there and discover something new every day.

From an engineering POV what has inspired me the most was the method with which the stresses/strains of the structure was modelled (without numerical methods or finite element analysis).

[0]: https://memetician.livejournal.com/201202.html

Metro networks, airports and universities are also very good examples of architectural structures that have been or will be built over many generations. Metro networks are particularly good examples because they truly are built from rock unlike pretty much any newer huge building in modern countries (Sagrada being an exception). Back in the days, only kings and powerful religious groups could build such structures.

It's a very different situation though, a cathedral can't really be used until it's finished or at least significantly complete.

A complete metro network might take 150 years to build but each line is useful on their own, the final network is composed of several semi-independent lines which get added and expanded over time, and even if the system's fulness is designed at its origin with multi-decade plans it's useful long before that.

Same with road & highway systems, or other public transport networks (trains, tramways, …). They're useful and have a point long before they're "finished", assuming there's even such a thing.

Many medieval buildings have developed in the same way, though. I don't know if it's true for the Notre Dame, but plenty of cathedrals and castles started out much smaller than they are today, and saw regular expansions over the course of centuries. And that iconic spire of the Notre Dame was added in the 19th century.

From what I've read, the spire that was added in the 19th century was replacement of the one that had been removed a century earlier