Questions about npcs and what makes a good map

and what do you look for in a portal map - how do you think of good puzzles for portal?
finally I can't find the tstchmb15 or whatever it was in my sourcesdk_content folder - infact I can't even find a portal folder in there -any help? Thanks.
Oh yeah, and Hi 
As to your concept for the turrets versus combine, the thing to remember about Portal is that it is all about portals. What would make Chell placing turrents while she holds the ASHPD more interesting than Gordon Freeman placing turrets with the ZPEM (Gravity Gun)? How would it require you to ... well ... think with portals?
As for how to think of good puzzles, I can't help you there. Some people just have a gift.
As for the sample map, try verifying your game cache files. The map was not automatically downloaded for all people during the patch, and since it isn't needed by any program, Steam doesn't really care that it's not there.
I like the idea of placing turrets to fight off the Combine, and in fact, I've been planning something along the same lines. The Portal part of the gameplay would be that you would have to find the most effective places in the environment to put the turrets, and you'd have to move around reactivating defeated turrets or deploying new ones, all the while avoiding Combine gunfire.
I don't think we can do this yet with the materials Valve has released. When they update the SDK Base (which contains the AI code in a readable/modifiable form) I think we'll be able to do it. Should be a month or two before they release a new SDK Base, maybe longer.