[TheNewt]

portal_pixie, if you are still stuck on 18 advanced 'eres a spoiler:
In the begining, shoot a portal into the ground and one right above you, you may have to position yourself while falling to land in the portal again in order to launch the other end. While in mid air, shoot the higher ledge's ground that you won't be able to shoot at after you landed. Then shoot a portal into the wall and jump through to get into ledge. When on the ledge, shoot a portal above the little black square on the far side, it may be hard to notice but shoot right above it and step onto the ledge.
After that shoot through all the pillars to the far ledge to the right, and just step into your previously made portal behind you. Continueing on, you should shoot a portal near the roof of the map, and jump into the portal you just made and you will fall down on the ledge across from the button, create a portal beneath you and launch yourself to the opposite ledge.
Go inside and open up the gate thingy, when inside you will see 4-5 turrets. This part had me confused for about 1 minute, before I just started launching my self behind each of the turrets and picking them up and throwing down to the tar/sludge/toxic liquid. I started with the one directly to your right as you enter the room, then the one farthest away from you to your left, jumped over to the one next that one. Jumped back to the ledge, and then created a portal to the far right one, and threw him down. After doing that I went back to the ledge, took the lift over.
I jumped down to the button, pushed it, opened up a portal on the now opened gate on the other side of the room from where you came, and one on the platform below you, jumped into the portal on the lower platform launched out of the gate, flew over, went into the lowered platform AGAIN, and launched for a second time to get over to the cube (that one was the most fun), grabbed the cube and went through a portal to get over to the other side of the room, if the gate closed by the time you finished this, just open it up again. Then the rest of the map should be the same as the main game's version of level 18, though the platforms later will dip down into the liquid. Theres 2 ways you can do that part, which is 1 portal back down to the start and use that hole and continually launch your self, or attempt to do it all without landing on any of the platforms (its harder but possible, I did it on my first try. ) Have fun! A note, I wrote this from memory, and got tired of writing after half of it. Sorry for any typos or confusion while reading it, there are videos you can watch that will show you how.
PS: I never watched any of the videos, I solved all the puzzles by myself.

[Twilight Suzuka]

Anyone who watches videos to figure out how to do it is a communist fool.

[Lord_Destros]

What about those who watch videos to figure how NOT to do it?

[TheNewt]

Anyone who watches videos to figure out how to do it is a communist fool.

I'm not a communist fool then, woohoo.

[purple_pixie]

I don't plan on watching any videos, myself ;-)

I didn't really spend much time on 18, I just got onto the third platform ( the one just round the corner from where you start ) and looked around a lot before getting the feeling there wasn't a single sodding thing to portal onto.

I'll give it another go soon, but I'm playing with C++ atm, and a bit of TF2.

Now you come to mention it I don't think I finished 17 ... I got up to flinging through a hole in a glass wall before I had to go out somewhere.

Love portal.

[TheNewt]

Theres a notch in the far corner you can portal onto, its gonna be black, and small.

[purple_pixie]

I've done it now.

I'd found that notch and had about 5 tries at landing on it, and then assumed it was impossible and tried to find something else.

Soon as I knew that was where I was actually meant to go I managed it ... arrrgh.

I love taking out the sentries later ... it's so ninja :-D

[TheNewt]

Yup, easy tho miright?

[purple_pixie]

Wasn't particularly hard, no.

At least, once I decided to do it in the right order :-D

[[ --<-@ ] Black Rose]

http://www.youtube.com/watch?v=RHtz6Q_JwNg