Here's a little bit of history:
After Trine 1 and 2, indie developer Frozenbyte tried something new, moving their flagship 2D platformer series into the third dimension. They succeeded in making a great game, but this decision ultimately led to the game's failure. The move to 3D proved far more expensive than expected, and Trine 3 was released into early access in order to alleviate the costs of development. Despite this, the devs risked bankruptcy if they continued to work on the game, so they cut their losses and released the game unfinished. The final product ended up missing some features (eg. there's only one difficulty level) and only had about half of the content that was originally planned.

What happened during the game's development is really unfortunate, because underneath all of this the game's quality shines through. Trine 3 is gorgeous to look at, even by today's standards, and the music is just as great as the previous entries in the series. The characters and story are revamped, improving on the already decent plot of Trine 2 with better writing and voice acting. Levels are well designed, movement feels good, and physics puzzles are clever and have varied solutions.

The game really could have been great, even surpassing the excellent first two games in the series. But what we got is half of a game, which ends abruptly in a terribly unsatisfying ending. I would recommend this game, but what you are buying is half of the game it could have been. At 50% off then, this game is well worth the price.