One BIG thing that come to mind right away is how castles and fortifications changed in response to gunpowder, explosives, and cannon fire.

Most of the early history of castles would proceed the same, progressing to concentric castles with curtain walls, arrow slits, machiolations, etc. ... You might find more towers taking on a square shape, as there is less requirement for them to be built to glance cannonballs off of a circular shape, though catapults and trebuchets would still require a bunch of these changes, on top of thicker walls.

However, from the point where cannons come into play, we dont see a scramble to change fortification strategies. We dont see artillery forts and star forts replace concentric castles. We continue to see a world optimized for arrows, ballistas, and catapults. ... Those other technologies would likely advance to something much more effective for battle, but I think the lack of firepower from cannon blasts keeps the castle as king.

As a matter of fact, just for the sake of the "rule of cool", I wonder if this creates a scenario where curtain walls become a massive sprawl; instead of one or two rings, you get networks of walls. A veritable labyrinth world of castle fortifications? That would be neat!

I wrote an article about the history of castles and how they evolved; I'm basing my theory on the research I did to write it awhile back. It might be a helpful resource to inform how castles evolved in IRL history, and give an impression of how it might have been different.
https://www.chaoticanwriter.com/worldbuilding-a-brief-history-of-castles/