The Fortress. It's been though many iterations. Began its life as a Secret Citadel. Really became prominent in the silver age, so far as I can tell. Indelibly crystalline and arctic to many, due to movie fame. Do have a preferred canonical version? A headcanon version?
I guess the first question is whether or not you think it needs to exist.
If you think it needs to exist, the question is what you think it should be. It was a trophy container and gym to start. In later years, it housed robots. And rooms dedicated to his friends. And later, it very Krypton-oriented. To be honest, I'm not even sure when its name became Fortress of Solitude. So, place to be alone? If so, should it shield from the noise of the outside world? Or should it be a reminder of Krypton? A prison for those too dangerous to hold elsewhere? A lab?
I think it's easier to decide how it came to be after you have a purpose. Did Clark build it out of earth-available materials? Or maybe from materials from the various aliens he encountered? Or from specs sent in his capsule (which lends to a longer planning time from Jor-El with likely knowledge he would not survive himself)? If so, what purpose did Jor-El have in mind and did he provide any of the materials or are they earthly? Maybe built from Kryptonian tech scavenged from time travel? Or do you like it grown from crystal?
Lastly, must it be surrounded by ice and snow for you, or do you like jungle, mountain, etc. versions?