Since I cannot review this part of the game (I cannot edit my shadwen review... or I need to contact the gog support just for that) I will review it here.
Escape from Castle at the beginning seems a lazy way to make the game longer... and many levels are the same of the main story, you just go from point B to point A instead of go from point A to point B.
There are no new cutscenes, no dialog, no new achievements, no point system (like in the demo)...
But in the last few levels (the initial levels for the story), you will understand what they really want to do... these levels are indeed much more difficult than the levels on the main story. Even if you decide to kill everyone and using all the tools you still will need to use your brain or you will get stuck with no tools in a small hallway with a dead guard on the floor and an alive not killable from the floor guards just around the corner.
The problem with Escape from Castle is that only few levels in the end are made in this way... and you still need to complete again many levels that you already completed 1 time (with not even an achievements unlock).

At least it is free and it is not mandatory... it is like a good mod. But I would have preferred some different level, or some more dialogue, or some more cutscenes.
Still I'm glad I played it to the end because the last levels are really a great rework of the original levels.