A bit of background for the game, and why a fortress is an impressive endeavor...
Food is important. You have varying degrees of Nutrition, and the higher your nutrition the slower your hunger bar fills. Fish are amazing for nutrition, meat next, and plants least, with water technically filling hunger but giving no nutrition at all, so you get hungry faster. For the purpose, "I want to be a fisherman" gives you 2 nets for passive fishing and a rod for active, and is the easiest start since you will assuredly have food, and that means more time to do other things.
"Hunting Trip" is nice too, since you start with a bow and arrows and you can butcher your father for early food (cannibalism is A-OK!) and this will last you a few days. If you start with bow skill, that's also easy hunting.
As for the castle? Wood buildings are the only option, and they take 6 whole trees per 1 wall segment. A modest 3x3 house has a perimeter of 20, for a total of 120 logs felled. Trees are about... 10-25% of the map, so in order to build a suitable fortress that'd be probably on the order of 800+ tiles scoured, which is roughly 28x28 area, and for all intents that's MASSIVE for just a modest 3x3 walking room house (5x5 including walls).
So for any proper fortress... good land management will be important to understand the size of the thing to fit into map tiles politely. This is actually important because each overmap tile has a limit on the number of items it can hold, so keeping things in different zones can help reduce item loss due to memory overflow. A farm zone would have to be in another map tile, for sure. Similarly, the smoking room (for curing meat) and drying area would probably be best in other zones, which begs the question of just how big this thing will be. If we're counting it in overmap tiles now, it should probably be at least 2x1 enclosed zone, with additional farmland zone(s) nearby, all of which would be enclosed in cheaper fences+spike pits.
This will surely be a massive undertaking, though I may suggest you actually not build on a mountain, but instead on some marshy-ish land. Certain lowlands where there's few trees, there's plenty of conveniently flat land, and there's usually herds of animals in the large open spaces, like reindeer and elk and stuff. That's good hunting, and there's usually pockets of water to drink from as well.