..... The wall must be continuous and unbroken when formed. If its surface is broken by any object or creature, the spell fails.
What exactly does that mean, though? Apart from the lack of (S) in the spell description, couldn't just looking at this sentence alone imply there's no reason I couldn't make an entirely vertical wall surface---even if one part of the surface is perpendicular to another? Again, this is a bit of a moot point sans (S), but still...
To me, it implies that---from a conceptual standpoint---I've got multiple (in my case, 16, having that caster level) 10x10 "pieces of wall" to place--the only restrictions on them are that they all have to connect to another piece of wall, that they must be vertical (as listed in another part of the spell's description), and that I can't have anything piercing through the area of the wall when it's cast. I can live with that, and at least as I see it, I stuck to that in the original intent of stopping the dragon.
Maybe the dragon should have been allowed a Spot check (
DC 40 to spot an invisible inanimate object) to see the wall and avoid? Not sure,
I can live with that.
...If he spots it, I'd look at his maneuverability, speed, and figure out if he could turn enough to avoid hitting it or not. Figure he may also have Wingover, which means, that if he spots the wall, it's actually easy to avoid.
Had the dragon missed the spot check, I'd have had him run smack into the wall and take some nominal damage (just a few d6s based on the collision and fall). It wouldn't be much, but it would be enough to get him out of the air for the time being.
I'd certainly not have a problem with the dragon taking a minor amount of damage akin to fall damage, but see next quote...
You placed the wall of force on your own turn I'm guessing? IMHO it will take a readied action while the victim is running to get someone to slam into a barrier they can percieve.
Therein lies the question as to whether the dragon can spot the wall.

If he fails the check, it's a barrier it
cannot percieve, and would therefore slam into it.
That said, I was indeed casting the spell on my own turn (specifically, I'd held my action until after the dragon had gone), so I'm cool with him not necessarily crashing into the wall and taking any damage. The intent was therefore to create the wall pretty much directly in front of the dragon, making it difficult to avoid the surface; not necessarily to cause it harm, but to impede it from carrying out its minimum movement to continue to fly based on maneuverability.
Dragon has blindsense and thus when it takes its turn, it knows the wall is there......
I go along with that. Again, knowing the wall is there only really matters if the dragon can avoid it....
..... If dragon has the
Hover or
Wingover feats, it can use one of those. Indeed I suspect dragons were given those insane fly speeds so they can hover as a default tactic.
If the dragon did not have
Hover or
Wingover then it would have to land at the end of it's turn or fall. If it was
really high up it would get a reflex save to stop the fall part way down
Wall of force does not need an anchor.
Wall of force does not give a save because it can't be used to trap someone. If you try using it to have a victim slam into it, how that will work is up to the DM.
A dragon of Huge size has Poor Maneuverability, and that means a minumum of half movement speed during flight to stay aloft. It has to move in a 45 degree angle to go up or down, and turning a 45 degree angle requires 5 feet of movement. As the wall was to be directly in front of the dragon, it's not inherently the wall that stops the dragon; it's the dragon's maneuverability rating. A beholder of will-o-wisp, the two example creatures for Good and Perfect maneuverability, would simply need to expend more movement to go around the wall, or a Gargoyle with Average manueverability could just turn in place at a loss of 5 or 10 feet of movement, no biggie. Flight feats such as Wingover and Hover could indeed change this, but it was a chance I wanted to take.
To boil it all down: It sounds as though the Dragon would have done one of two things:
(A) Used Hover and/or Wingover, not crashed into the wall because it would have likely detected it, and just had to fly around/over/under it, or....
(B) Cursed its feat list, crashed (I'd say lightly, you might almost say "bumped into" instead) into the wall, counted all the rules dealing with falling 30 feet, and have been brought to the ground, even for a short time (which, in effect, was really all I was shooting for in the first place once I'd determined that I couldn't shape the effect).
Is that more or less correct?