Castle Aldrheim is where the Order resides, and it is also where the wizards first begin their journey to saving the world.
Castle Aldrheim is located just north of the Disappointing Forest and west of Boring Bay, close to the southern end of Midgard. The Jarn Mines are seen to the northwest. A river runs to the north of it, and there is also a bridge which is used to cross the little lake that the river makes before flowing towards Boring Bay. It is seen in Chapters 1 and 7.
Castle Aldrheim is a white castle with red pointed tops and surprisingly no drawbridge. It has three known floors; the bottom floor is a dungeon filled with switches, levers, imps and a giant masher which is completely useless. The middle floor is a kind of living room where some people are currently having a party. The topmost floor is shaped like a cross, with the northern end a dining hall; the eastern end a kind of reading room; the southern end stairs to go down; and the western end a schoolroom where Vlad teaches.
The Wizards of the Order of Magick are the sole inhabitants of the castle, but there is diversity in their attitudes towards being members. There are members who use their status to presumably put others down and placing themselves at a higher status, as seen with the Headmaster of the Order; members who use their status to do what they want (generally party and have a good time, listen to music and eat cheese); and members who take it seriously, spending time in the library researching and creating new magicks.
Occasionally (or maybe frequently), a member or members of the order will be called upon to embark on a journey to save Midgård from whatever happens to be the problem at the time. How members are selected is not stated, but it can be presumed it's something along the lines of "Eeni Meeni Miini Moe".