Over the last twenty years, computer games have grown from a niche market targeting young adults to an important player in the global economy, engaging millions of people from different cultural backgrounds. As both the number and the size of computer games continue to rise, game companies handle increasing demand by expanding their cadre, compressing development cycles and reusing code or assets. To limit development time and reduce the cost of content creation, commercial game engines and procedural content generation are popular shortcuts. Content creation tools are means to either generate a large volume of game content or to reduce designer effort by automating the mechanizable aspects of content creation, such as feasibility checking. However elaborate the type of content such tools can create, they remain subservient to their human developers/creators (who have tightly designed all their generative algorithms) and to their human users (who must take all design decisions), respectively.
This thesis argues that computers can be creative partners to human designers rather than mere slaves; game design tools can be aware of designer intentions, preferences and routines, and can accommodate them or even subvert them. This thesis presents Sentient Sketchbook, a tool for designing game level abstractions of different game genres, which assists the level designer as it automatically tests maps for playability constraints, evaluates and displays the map's gameplay properties and creates alternatives to the user's current design in order to speed up the creation process and inspire the user to think outside the box. Several AI techniques are implemented, and others invented, for the purposes of creating meaningful suggestions for Sentient Sketchbook as well as for adapting these suggestions to the user's own preferences. While the thesis focuses on the design, performance, and human use of Sentient Sketchbook, the same algorithms and concepts can be applied to different mixed-initiative tools, a subset of which has been implemented and is presented in this thesis.