Niffelheim

The are 4 Workshops total that you need to upgrade: Sawmill, Forge, Kitchen and Alchemy Station. Each of them have 5 Levels - Level 1 being when you first build the work station, then 4 additional upgrades to get to Level 5. If you need a breakdown the of the materials required for each tier, please see Great Architect.

You'll need to upgrade the Forge and Alchemy Station first as you'll need the items crafted on those stations for your Castle upgrades as well as Weapons and Armor. The Sawmill should be your third-in-line focus to upgrading and the Kitchen should be your last (you can buy far better food items from the Shop versus what you can craft in the Kitchen).

Upgrading your workshops and castle should be your number 1 priority in the game itself since it is the most time consuming task compared to actually opening up the Portal to Asgard. Due to this, you will get this trophy well before you reach Day 100 so just keep gathering the materials you need to build them up and once all four are upgraded to Level 5, the trophy will unlock.