Eragon's levels are broken up into two styles. There is the action/adventure aspect, which is most of the game, where you control the young farm-boy as he attempts to learn his new powers, become a Dragon Rider, and save the kingdom from the evil emperor. And then there are the levels where you control his dragon, Saphira, as she flys through enemy-infested landscapes.
The levels where you control Eragon are fairly basic and straightforward. You run around a fairly constricted level, killing enemies and collecting items. Eragon has three major weapons. He has a sword, a bow and magic. Since Eragon has just learned of his destiny, he doesn't know a whole lot of magic. Throughout the game, he will learn spells that can be cast by drawing their symbol on the touch screen, and then cast them with a flick of the wrist (See Game Mechanics).
Saphira's levels are simple and mostly feel a bit tacked on. Here you will have to navigate her through a series of rings along a pre-set path in order to avoid enemy fire. These levels are nowhere near as frequent as the Eragon ones, and whenever they occur, they just feel a bit out of place.
As for the game's storyline, I found it fairly hard to understand individual events because it seemed like the plot-advancing cut-scenes covered a whole lot of events really fast. Sure I got a good idea of the story overall, but plenty of the details were lost on me. I'm sure I would understand a bit more of the game's story had I read the book, but as a stand-alone experience, Eragon is a bit lacking.