Agatha H. and the Siege of Mechanicsburg is the fourth Girl Genius print novel.

It covers the highly eventful stretch of time from the moment deep under Castle Heterodyne that Zola mentally frees Lucrezia by removing Agatha's locket, through to Agatha's triumphant procession to the front steps of the Red Cathedral following the breaking of the eponymous siege.

Along with the usual swarm of footnotes highlighting various facts about Agatha's Europa, we are given more details about various characters, especially Vanamonde von Mekkan's self-appointed assistant, and the clank-copy of Lucrezia. (Including having the latter being explicitly present on Castle Wulfenbach, something that is never shown in the comic.) The ending is notable in that it veers off from the comic somewhat, and reveals several things which originally remained mysteries for much longer, including that Zeetha's treatment following her stabbing involves some pretty extreme measures, the fact that Klaus has placed a mental overlay in Gil, and that both Othar and Moloch are still in Mechanicsburg to witness the aforementioned procession.

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