Stone's fortress (starting enemy level 26) would be a little easier than Raze's (level 28). The entrance is on the NE end of the mountain island in the middle of the channel leading to the harbour from the start of the fjords.
At level 25, if you hover or go anywhere near a wizard tower (that shoots lightning at you) you will not last very long in Raze's flying fortress. If you keep moving, though, you should be able to make quick trips to the next section of the fortress, etc.
FWIW, when I played through as an archer, the first group of opponents in Raze's fortress brought me up to level 28.
