It is not a sandbox - you select missions, and assign pre-gen heroes to go. The mission maps, I believe, are randomly generated each time, so every play through is different... Except the heroes are the same and presumably the missions are the same. There is some customization in what abilities you select for a hero (set list, but they can be changed mission to mission) and you can upgrade their gear. You can also vary the mission difficulty for more XP and awards.
It is NOT a baulders gate/diablo experience - this is turn based.