"Thief II" was built with the third iteration of the Dark Engine, which had been used previously for "Thief" and "System Shock 2". According to Pearsall, the Dark Engine had become "a very well understood development environment", which made for an easier production process. Engine updates created for "System Shock 2", such as support for 16-bit color, were carried over to "Thief II". The average character model in "Thief II" was given close to double the polygons of the average model in "Thief", with much of the added detail focused on characters' heads. This was an attempt to give the characters a "more organic" look. Certain artificial intelligence (AI) routines written into the Dark Engine, which allowed enemies to notice changes in the environment such as open doors, had not been used in "Thief" or in "System Shock 2 "but were implemented in "Thief II". Weather effects such as fog and rain were added, and technology from "Flight Unlimited III" was used to generate the sky and clouds.