For the fortress itself: Have a graphic in the background of a snow fort being built.
Take each of the Seasonal Items and make them into a snow brick and line them up side by side in a row. As you complete a Mastery for an item, a new brick is added to the appropriate column for the level of mastery. (3 total bricks for Tier-3).
To build a brick, you need to have Snowballs (the amount changes as you go up in mastery), some kind of carving tools (e.g. picks, explosives, etc) and the mastery required for that brick.
Once you complete an entire row of bricks (all of tier-1 for example), you'll start receiving dailyfrozen nets daily from Buddy
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That he happens to find lying around. The higher your Fortress is the more Frozen Nets Buddy can find.
Once all rows and columns have been completed, the bricks will now resemble the walls of a Frozen Fortress. This graphic should be viewable from a user's profile.
Once the season is over, the Fortress begins to melt. Buddy will no longer be able to find you Frozen Nets once the Fortress is melted.
"Frozen Fishing Fortress"
Buddy was bored. Fishing was fun, and winter is fun, but Winter Fishing alone wasn't.
Buddy decided the thing he needed was his friends, but how was he going to get them to join him. A snow fort on the water was a fun idea, but it would have to be BIG and BEAUTIFUL to make Buddy happy. That's where YOU come in!
Buddy needs you start fishing in all the waters around Redbrook and start mastering the Frozen Items you find.
Once you've mastered a Frozen item, go to Buddy with alot of snowballs. The walls of Buddy's "Frozen Fishing Fortress" must be made of dense snow and ice, with motifs of the Frozen Items carved into them. (Buddy said you should do all the building and carving. He feels his talents are better used elsewhere, but he believes in you 1000%)