Complex sentence: The game area was in real 3D, and monster/enemy AI had free movement around each level environment, unlike in other games of that time, running from monster/enemy AI up some stairs meant they couldn't chase.
Split into simpler sentences: The game area was in real 3D, and monster/enemy AI had free movement around each level environment. Unlike in other games of that time, running from monster/enemy AI up some stairs meant they couldn't chase.