Euclidea is a puzzle game where you have to try to solve different geometry problems that gradually get harder and harder. At the beginning you have to solve a dozen problems in the tutorial mode, where you learn how to chart circumferences, make straight lines, and calculate angles. Later on is when things get complicated.
In all, Euclidea has 120 different levels with a steep difficulty curve. Solving the first few may seem relatively simple, but as you advance, you'll run into some really complicated problems that are only really possible for people who have studied math.
One of the strong points in Euclidea is its excellent control system. The game is developed with touchscreens in mind, and it shows. Interacting with the puzzles and using the different tools is comfortable and intuitive.
Euclidea is an excellent intelligence game that, while geared toward people who have studied math (specifically geometry), has the potential to entertain anyone willing to accept a new and difficult challenge.