EDIT: The "Plus" version is already on GOG, so this entry is probably redundant. Original comment follows:
This game was mind-blowing for its time, with gleeful freedom of movement and the ability to reshape the very terrain of the levels via increasingly powerful spells. You build your own fortress, seeing it rise procedurally from the ground in response to your magic, and as your power grows, so does your base. Eventually, you can unleash apocalyptic attacks on your rivals. In fact, the feeling of gradually increasing power is what stuck with me the most. It's a rewarding feeling, and Bullfrog nailed it here. I never heard of the "plus" version with the extra content, but if GOG can resurrect that version, all the better!