When did invading a fortress via a sewer become popular?I do not know precisely, but there has been a recent surge. Between late 2011 and mid-2012, I saw three Sword & Sorcery movies that employed this tactic (details below). Two of these involved the protagonists breaking-in to their infested homes. I think the originality has been exhausted now, and I hope this fad fades away.
Star Wars IV: A New Hope 1977
Death Star Fortress- Sewer Exit-Escape
George Lucas deserves credit for cinematically popularizing a fortress escape sewer-scene in his 1977 Star Wars IV, A New Hope (Death Star garbage compactor scene). Ostensibly, Star Wars is Sci-Fi rather than Fantasy, but it employs most every Fantasy trope that exits. It certainly seems to have inspired the 2011 Conan the Barbarian break-in into Zym's tower (below).
The Sword and The Sorcerer 1982
King Cromwell's Castle: Sewer Entrance
The hero Talon sneaks through the sewers to rescue potential heir to throne Prince Mikah and his sister Alana.
Conan the Barbarian - 2011
Khalar Zym’s Castle: Sewer Entrance
More special effects were dedicated to the randomly-placed, lurking sewer-serpent "dweller" than were used to demonstrate the power of the magical mask of Acheron motivating the story. So strangely, this scene was more exciting than the climax of the movie (Here is a link to my rant on that).
Solomon Kane – 2009 globally (2012 in USA via Video Streaming)
Josiah Kane’s Castle: Drainage Entrance
I enjoyed this movie (Link to review) , but I took a sigh when this sewer-trope was demonstrated.
Snow White and the Huntsman 2012
Ravenna’s (and Snow White’s) Castle; Sewer Entrance
Snowwhite had escaped the castle via the drainage tunnels, so when she returns with and army, she sends some dwarves in to raise the gate. Very clever...I suppose.