The setting is a fantasy world with many races, magic, and 11th to 17th-century technology with a very small portion of steampunk, mostly airships and some downgraded steam-ish technology. The magic is slightly weaker than in most other fictions. There are very little "offensive" spells, like fireball or ice storm, instead magic plays more of a supporting role in warfare, by healing, enchanting melee weapons and armor, creating golems, preserving food, a little bit of weather control, creating new armaments, sometimes conjuring or controlling beasts, etc. What would castles, fortresses, keeps, bastions, citadels, etc. be like in this world?
Keep in mind that most battles are more medieval-ish, guns and cannons are slightly 'nerfed'. There are beings (dragons, pegasi, harpies) that can fly. Although most of them can't go very high or maintain flight, for it is very tiring. They are more of an 'elite' regiment among armies, almost like knights, have very small numbers and depend on ground support most of the time.
Minerals/stones are an unlimited stock in the world. Transfusion or creation of these materials isn't viable though since it requires a lot of skill and produces very little. Instead, this world has "inexhaustible" mines, which have a pretty long explanation of its own, but the main reason for this is just so the world doesn't run out of stuff like iron and marble.
As this question was flagged as opinion based, not saying it was wrongly done so, I decided to address the justifications made by those who closed this question, well only some of them actually, only three of the five guys commented their reasons.
First, there's no abundance of resources in this world, be it magic or materials. They are indeed "unlimited" to an extent, but that only means that the supplies used by these civilizations won't run out as times passes. At no single moment, there will be an infinite or abundant amount of materials in the world, not much more than there was in our world. I simply don't want this world to, well, run out of things like iron because all the mines are depleted.
Second, I have a pretty basic grasp on medieval Europe. And this is one of the reasons I'm asking about fortifications design here. If I say something wrong, please correct me.
Third, my world is not completed yet, nor its rules. That's why I'm here, the website's name is "worldbuilding" after all. If there's an information that is crucial to answering this question, then let me know, I'll provide you with it or at least seek advice and suggestions. Also, if you have any ideas, please let me know, I'm all ears.
Fourth, castles are not obsolete. When cannons were invented, they did not become obsolete, that's factual. And as said before, the airships and flying beasts do need help from the surface in order to besiege a castle and magic here is not used offensively, many mages simply won't storm a fortress with little help from conventional means.
I'm looking for answers on how castles would 'adapt' to this world, so it is obvious that the standard castles wouldn't be effective.