In my world, there is a magic civilization. They are on a far away continent, but it is harsh and snowy. In the current time, they have now giant cities, the main ones being a mix of both steam and magipunk. For worldbuilding notes:
The continent they live in has a lot of similarities to that of Siberia, but the further North you go the more it starts to be like Antarctica
The coast is fertile, and the higher up North there are lots of mountains and eventually icebergs
The magic system is energy-based, and is used for mass-heating, powering cranes and other machines, and curing illness
The settlement was created back in the 1200-1300s. The current story takes place in the 16-1700s
The wizards and witches brought along not just aristocrats but also carpenters, blacksmiths and welders, and carpenters along on their odyssey.
Due to conditions, the local natives (based on Vikings and Eskimos) will not be a problem. They ended up either becoming part of the civilization or eliminated by magic warfare
Where they built this new city there was a previous civilization that had been mysteriously abandoned. Not much is known about them, but the walls and sewers are still intact
The architecture is mainly based on Rome (as a whole kingdom, architecture varies, but this is for the capital)
My Question: With all these conditions, how could a sustainable city in such a harsh environment be built completely?
If I am missing other factors, I will also accept those too.