I love the background in this depiction of "the Fear Door". The light and shading is very well placed. It really gives the impression of being in a dark, underground secret passage. Those crystals have great texture and a kind of threatening tone about them, probably because they're so sharp. The arrow-shaped door hinges are a neat detail, too. Way to point our two heroes in the right direction.
Twilight and Spike's expressions are exactly what we think of when this scene comes up (or at least for me they are). Spike's tears and thankful smile as he hugs his best friend, who means so much to him that the very idea of losing her is the essence of his worst fear. Twilight's posture as she kneels to hold her first friend, comforting him and resolving his fear, saying she will never send him away. Once again, the light on these two is fabulous. I can't say that enough; the light really sets the mood and atmosphere for this scene.
This scene was particularly heart wrenching because it showed how much Twilight means to Spike. He's been with her since he hatched (thanks to her) and has come to view her as his best friend, maybe even a sister. She means more to him that he normally shows; it's only at the thought of losing her that his true feelings come to light. I wonder if this fact about Spike will turn up later in the series....
Anyway, great take on this memorable scene. You really captured the heart of it.