How would magic effect medieval warfare?

Specifically 5e D&D magic just because it's what i'm the most familiar with at this point.

Huge armies and formation fighting is basically worthless as you now have walking artillery that can shoot at several hundred feet, that take as much resources as any other foot soldier.

Castles are really only valuable as a point with which to center your magical defenses, as walls basically only give the illusion of safety due to spells like fly, knock, invisibility, and passwall. Anyone that's really trying to protect their lives or their possessions probably has them locked in a demiplane of their own creation, or a secluded refuge on the prime material plane that they can only access via teleportation.

I think high level magic users are really the only deciding factor in who would win a war. They are able to teleport to a city and destroy the entire thing in a night. The only thing that can even kill a wizard of this caliber is another wizard of this caliber.

Personally I think that fortified defensive structures (like castles or forts) are probably way overdone, and the idea of having a fantasy army like from LOTR seems almost nonsensical. Most fighting would probably be done in small, extremely skilled groups of individuals who could work with the magic of a skilled mage who could teleport them into places.

That being said i'm curious to hear if you guys agree or disagree. I'm trying to homebrew a more cohesive world and get rid of fantasy tropes that don't make a lot of sense.