A turret on a ledge is only useful when there's another ledge opposite it. Otherwise all the alien has to do is walk down the bottom of the ledge, since turrets can't fire past about a 45 degree angle (just approximating) downwards.
A
| |
|1 2|
A = turret, 1 & 2 = aliens, | = ledge (side view)
Here, we can see that turret A can fire on alien 2, but NOT on alien 1, as its vertical pivot can't go down that far. Turrets on one ledge are virtually useless, unless they're intended to be protecting from something from the other direction (i.e. the ledge is a safe area).
A B
| |
|1 2|
A & B = turrets, 1 & 2 = aliens, | = ledge (side view)
This is where turrets on a ledge are useful. Turret A can fire on alien 2, and turret B can fire on alien 1, so the entire area is covered. As long as they're not placed close enough to be goon killed, this defense works pretty well, a good example being A.T.C.S. (especially if you block off the staircases with armories) The only drawback is that wall-crawling aliens can still get up to the turrets, which is why you need a good 3 turrets per side to kill off those Basilisks and possibly Marauders.
As for the guide, I found it was more-or-less obvious information. A better way to structure it would be to take a map, and do a walkthrough about how one would defend it. For example, you could go over each place to defend in ATCS, and which layouts are effective and which aren't. Heck, you could even mention whether or not certain areas are easily defendable places to move the reactor to. Something like that would be less obvious and far more useful. Also, watch those spelling mistakes.
Not that I mean to say this is BAD, it's still useful, especially to newbies, but there is room for improvement.