For the world I am designing, magic is not uncommon in it's use in the various population centres, some with advanced ability are typically in high ranking positions but not from a power standpoint, more in a city planning and logistics position. For example I have a city in a clay desert that was built by magic users by moulding the clay to form the tiers and the dwellings, some use control water in shifts to make sure the water flows smoothly and some use it to aid the flow of sewerage out of the city. Magic in hese places is a tool to be used to better society and if misused, there are consequenses.

This brings me to the city that breaks these rules. The magical city that is responsible for the research and development of spells to further their own standing in the world, they have their own rules that include the standard, no killing, no stealing, no non-consensual testing etc. There are some caveats though. If you can make a case for it to argue that it was necessary to further the development and understanding of the arcane, then you may be forgiven and even allowed to continue in some cases.

What does this have to do with Castle design and defense you ask? Well, be patient and let me finish!

This magical city is situated on the saddle of a mountain range, between two peaks, very high up, something something everyone is beneath them yadda yadda. Of course they have a castle up there because castles are a sign of power. They have unlimited use of spells, they are unlikely to use manual labour but summoning is fine so long as there are no ethics violations (they genuinely care about consent).

They are accepting anyone of all walks of life who wish to learn the arcane ways, after a lengthy vetting process and a simple aptitude test they will welcome anyone. The society is very accepting, some openly others through apathy. Not everyone is good, in fact some are straight up evil, but so long as they follow the rules, they are allowed to practice magic in the safety of the city.

There is everyone from giants to gnomes, celestials to fiends. Not everyone fits their stereotypes. There are even non magical beings, usually for consensual testing, like a barbarian who caught the eye of a wizard, trying to understand how their rage works, is it arcane? He vows to unlock it's secrets.

How would you design a castle for a magical city that:

1. The city is locked to this plane (no plane shifting funny business, they aren't cowards), though individuals can come and go as they please.
2. Can be defended from any angle (yes even plane shift)
3. Could fend off a Dragon if one decided to attack.
4. Could be infiltrated by some truly out of the box and questionable thinking by a totally mismatched adventuring party.
5. Has a code of ethics that outlaws torture, gives everyone a fair trial, would rather turn people away than capture them but would still brutalise an opposing army provoked.
6. These guys have a minister of defence and a military force but on the whole they are not a homogenous society, there are many who are in it for themselves, there is no conscription, the military is entirely volunteer based.
7. I wouldn't say they are rich economically, they just have magic to offset it. Their influence in the world is tenuous, some cities have a good relationship and trade network, others do not trust them at all.

Anyway I am getting off topic, tldr; I am designing a castle that can be defended from any assault, and has all arcane spells at their disposal (save for a few they deem unethical). Any thoughts would be greatly appreciated.