Castle Heterodyne

Castle Heterodyne is the ancestral home of the Heterodyne Family. Imbued with the charming personality of one of Agatha's mad scientist ancestors, Faustus Heterodyne, it is self-aware and considered to be alive. The castle was nearly destroyed 19 years prior to the beginning of Girl Genius in a mysterious and as-yet-unexplained attack by The Other. Many people were killed in the Castle that same night, including Agatha's infant brother Klaus Barry Heterodyne, her grandfather Dr. Mongfish, and the son of Carson von Mekkahn.

Being sent to Castle Heterodyne to assist with repairs is considered the ultimate punishment detail under Baron Wulfenbach's rule, as the chances making it out alive again are extremely slim.

Spoilers
Following Judy's last instructions to her, Agatha makes it to Mechanicsburg and speaks to the broken Castle for the first time. At the end of Volume 6, Agatha finally enters the Castle in a quest to repair it and prove that she really is the last Heterodyne. This will be quite a challenge, given that there are still dangerous prisoners inside, a fake Heterodyne who wants to kill her, and that the Castle can't control the innumerable death traps still active inside.