The code was completely different from the original Quake engine, and would require throwing away eleven months of work for a complete rewrite.

The game itself was a commercial total conversion of the Quake engine.

Since its release, the Quake II engine has been licensed for use in several other games.

However, already behind schedule, the decision was made to port the entire game to the Quake II engine, six months into development.

This technique is found to have worked best in Quake 3 engine based games.

MD3 is a model format used by the Quake 3 engine as well as its many mods.

Blood was ported to the Quake engine under a project called Transfusion.

However, programmers taking advantage of these changes lost backwards compatibility with the unmodified Quake engine.

Mainly they're not interested in all the work required to support the licensees, after their experience with the Quake 3 engine.

In true 3D engines to follow, such as the Quake engine, room-over-room became an easy effect to pull off.