Portal is not a particularly challenging game control-wise. It is, however, more challenging mentally than others, albeit only in the later levels. Completing Portal requires a mind which fully understands how portals work and can apply the physics behind them to any situation. It also requires good control skills (for those awkward jumps), and only rarely does it require good timing.
The game is split up into twenty different test chambers, with the difficulty rising in a logarithmic fashion. After chamber 19 (actually the 20th chamber, since numbering starts at 0) is complete, there are three additional levels.
Once the game has been finished, you can play the advanced and challenge chambers. There are six advanced chambers, which are ordinary levels with extra obstacles and challenges added to make them that little bit more impossible. They are unlocked as you complete each corresponding normal chamber. The challenge chambers are chambers 13-18 with extra challenges added, and the objective for completing them is one of the following:
- Use as few portals as possible.
- Complete the room as fast as possible.
- Finish with the fewest number of footsteps possible.
Throughout this walkthrough, you will find a basic solution; one that seems to be the expected solution based on the way the chambers were designed. In every case, this is by no means the only solution. This game was designed to make you, the player, be creative, and so there are often many other radically different ways to complete a particular puzzle, often involving more advanced techniques. This ability to be creative is something that has made Portal such a popular game.