Slayer le Boucher - B&C Forum
Fluff in White Dwarfs says that Magnus took with him 9 Silver towers to assault the Fenris system, so my gues is that Silver Towers has become something of a spatial assault/battle station.
Because only taking 9 of them, is just not enough if they're vehicles that you could play even for Superheavies, it makes much more sens if they're the size of the DA's Rock...
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In the WD its said that the Towers are phasing between the Mortal realm and the Warp, they used this to there advantage to go past the Imperial blocade that surrounds Fenris, while its been bombarded.
The Towers landed over specific Geo-thermal localisations and the Sorcerors and acolytes in them started to perfom bloody rituals.