ToME WikiI'm going to go out on a limb here and suspect that when a Heavy Crossbow of Siegecraft "renders you immovable", it's not supposed to let you move like a Death Mold does (i.e. short-range, semi-directed phase doors instead of walking), but instead make it impossible for you to move from your current location (except possibly via teleportation?).
Incidentally, that should probably be "renders you immobile", not "immovable".
-Derakon