[tssva]

Even the ones still standing often aren't because of fantastic original engineering but because they are the ones people have cared to carefully maintain and to perform renovations on to compensate for original engineering flaws.

[bdamm]

And sometimes simply by sheer mistake; the leaning tower of Pisa was supposed to be supported by the Corinthian columns and limestone internal blocks, but it turns out that its structure is actually supported significantly by the marble façade.

[geowwy]

An example is the Hagia Sophia which had a lot of reinforcements added after the original design was found to be unstable. The building you see today is not how it was originally designed to look, not just because of the minarets.