Level 10[edit]
Thought that Portal was the first first person puzzle game with portals? Think again. This level is a quick introduction to portal doors followed by a small portal maze. The switch to open the exit door is easy to find, but you might be surprised that the exit door is on the ground level.
Level 11[edit]
This level introduces rocket turrets that aim at you and robots. A simple circle strafe around the turret will make the rocket destroy its own turret.
Get rid of the first turret and the robot will hit its bin and unlock the door forwards. Repeat the same trick but with three turrets for the next section.
The final section adds in some robot guidance with toggleable walls and floors. Get rid of the turrets and it will be a piece of cake.
Level 12[edit]
This level introduces gelevator mazes. Just look before you leap and it should be no problem to jump onto the platform with Craven's face on it to unlock the door out.
Level 13[edit]
Another robot guidance level. Just toggle the walls infront of the robots and they will all end up in the central goal.
Level 14[edit]
Another floating platform maze, this time with a central tower. On the tower is a guard and the switch for the exit door.
Level 15[edit]
This level has you take manual control of ball turrets. Just stand on the indicated spots and 
to fire.
Level 16[edit]
This level has you flying through rings. Given how fiddly flight controls can be, you might want to skip this one.
Level 17[edit]
This level has you direct colored balls in pipes. Some pipes have arrows on them to redirect the balls as they travel. Each such pipe has a switch to toggle the direction of the arrow.
The first part is a quick check that you get the idea. The next gives you a big mess of tubing that is several stories tall. Just work backwards from the goal bins and it should be a piece of cake.
Level 18[edit]
Remember level 6? This is a remix of it. This time it is the second and fourth door that leads forwards. And the ball runs now have gaps that you need to jump over. Like before, use quicksaves liberally and you will be done in no time.
Level 19[edit]
Remember level 3? This is its bigger brother. Now the trench has two rings around the goal instead of one. There is also a bunch of red herring walls and floors. Just work backwards from the goal and you will have the solution in no time.