This is a really interesting card, and one that I think might be conveying a couple of primary concepts at once.
First, based on that quote from the LWB, I think it's putting a spin on the RWS 7 of Swords. That deck's version is "the theft card," showing a man making off with five out of seven swords pictured, and looking pretty shady about it. I think that card gets into notions of stealing from people in terms of intellectual/mental "property." The Sweet Twilight's version seems to focus more on defenses people build against those kinds of theft: that's what I think the wall represents.
But then there are the two "fairies," if that's an acceptable word for them, and the way the one in the foreground seems to have dragonflies and butterflies and even some sort of other fairy-creature growing from several strands of her hair. It suddenly occurred to me that the other fairy -- the one we can see in full at the upper right of the card -- might be the same size as the one at the lower left, and just farther away from us, which is what I first assumed...or it could be that she's just as close to us, but she's actually much smaller. It then dawned on me that it's possible she's even yet another creature that originally grew out of the hair of the fairy at the lower left. She might be raising her arms in simple joy at having matured enough to split off from the first fairy and become an independent being, or she might be cheering the breach in the wall (and she does look like she's cheering to me, and I've always assumed her wings mean she can fly, so I never thought to question if she might be falling, as Hemera wondered). But what I'm getting at is that she might be a companion and equal to the other fairy, or she might be more of a sort of offspring.
My latest idea is that the card is partly about the way our intangible thoughts, once we have them, can then become tangible reality. Maybe the second fairy did grow from the hair of the first and then separated into a wholly separate, sentient individual. Maybe the dragonflies and butterflies are in the process of doing the same -- maybe they all represent thoughts that the first fairy has had, and they're manifesting in the physical world around her now, literally growing right out of her head.
Similarly, it could be argued that the swords in the card show a variation on this theme. There are two pairs of swords on the wall that look to me like they've been painted there. Not far from each of those two pairs of swords, a hole has been punched in that sturdy-looking wall. This could be symbolic of the way that applied thought can devise ways of breaching almost any barrier. Also, in the topmost hole, there's also a real sword visible sticking through the breach. One way of looking at this could be that the second fairy used that sword to punch that hole in the wall, and that's what's making her cheer. A second way of reading this is that the painted swords resonated enough together to not only blow that hole in the wall, but they maybe even manifested a real sword in their likeness, much like the first fairy might be manifesting physical beings. This could be taken as a statement about how symbols can conjure the things they represent.
And then a final idea that's possibly being presented here is the notion that our thoughts can furnish us with a variety of ways to attack any problem. For instance, a fairy with wings can surely just fly up and over a wall...but maybe that's not desirable, or it's too tiring or too slow, or it's just good to have options. The mind can then offer alternatives -- like coming up with ways to go straight through that wall, instead of taking the long way over or around.
I guess for me, the card then has a few major interpretations all going on at the same time, any of which might figure into a reading: building defenses with our minds; the way thoughts can gain physical form; and the fact that the mind can deliver multiple solutions to any problem.