Yes i would have to know what he used : BSP and models ?
Some custom view/culling system ?
What optimisation tips has he used.
How does he managed ennemy AI ? (launch ennemy action only when player is near ?), what AI system to search for player ?

That's sad normal mapping seems to not be used in levels , perhaps it was too much power consuming to keep a good neought frame rate ?

Anyway this is a GOOD COMPLETED Indie game