Winter's Gate is brilliant. I've found previous Insomnium albums to be a bit pedestrian (albeit very competent vanilla melodeath). With Winter's Gate Insomnium introduced some prog & black elements without abandoning their core melodeath sound.

IMO there were two other outstanding albums released the same year that accomplished a similar feat - Be'lakor's 'Vessals' end Wilderun's 'Sleep at the edge of the earth'. Wilderun tend to get pigeon holed as a folk metal band but I don't think that accurately reflects their sound. I would recommend both albums to anybody that enjoyed Winter's gate