"Fortress of Solitude" is a famous phrase from Superman.
Lisa is comparing Nick's Toronto loft to Superman's arctic retreat, and thereby comparing lonely vampire hero Nick to lonely alien hero Superman, though of course she cannot know the breadth of applicability in her parallel. Lisa could have picked up the phrase directly from the comics, or from the Christopher Reeve movies, which she would have seen on TV or VHS given her age, or any other Superman stories she had encountered, but she must surely know she's citing Superman canon, junior fangirl as she is.
Additionally, as the creators of Superman were Canadian, there may be a tongue-in-cheek element of national proprietariness on the part of the writers in employing that particular phrase in that way.
The "high-tech dungeon of doom," as Schanke calls Nick's loft, has the most attention paid to its "Fortress of Solitude" element in the first season, with the alarm system, camera, and shutters. Later, this all seems more taken for granted. The characters and the writers alike got used to it, perhaps. Or does Lacroix's return affect the role of the loft in the story?