whereas Quake, HL (okay, Quake

) & Lith levels are built room by room out of voids, Unreal starts with a huge block & the level is carved out & then prefabs are placed...
its the main reason why Unreal is so good at rendering open spaces... the block you start & the amount you can do with it is based on system memory & engine limitations...
In Worldcraft/Radiance/Lithtech you can just add as many rooms & corridors as you want, you just get horrid r_speeds & load times
or you just break the engines' parser & crash the game

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Temporal Mechanics