This image is perhaps almost to dark in its representation, although there is certainly a dark thread running through the novel. However this simplistic representation of smoke and mirrors might be better aimed at the more mature spectrum of a YA readership. However that said I think the blue smoke could work well with the castle turrets perhaps edging out of the top. with the text layers then on top and centered.
6 Dry Ice Experiments To Do With Kids - Dry Ice Bombs