Between Turkish camp and Christian city was a belt of what a naïve person would identify as empty (albeit curiously sculpted and chiseled) terrain. Jack, a trained professional, by squinting and tilting his head this way and that, could imagine that it was as densely crisscrossed with sight-lines and cannonball-arcs and other geometrickal phant'sies of engineers as the space above a ship's deck was with ropes and rigging. For this corridor between camp and fort had been claimed by the engineers – as anyone who stepped into it would learn in as little time as it took a musket-ball to cover the distance. The Engineer-Empire, Jack'd been noticing, waxed as older ones waned. Just as Turks and Franks had their own styles of building, so did Engineers rehearse, again and again, the same shapes: sloping walls, backed up by earth (to deflect and absorb cannonballs) laid out in nested zig-zags, a bastion at each corner from which to shoot at anyone who tried to climb the neighboring stretches of wall. Oh, Vienna had a traditional pre-Engineer wall: a thin curtain of masonry, crenellated on top. But that was nothing but an antiquarian curiosity now, enveloped and shamed by the new works.