You can do part of the fjords before taking over the battle tower; a barrier prevents full access, which you will not be able to lower until you can turn into a dragon.
If you have not done much on Sentinel Island, I'd suggest reloading. You can get a few levels pretty quickly in the fjords, and that way you can make sure you get a full level's worth of experience from the island mindread (if you were 13 and half way to level 14, for example, you would have only got half a level worth of experience).
I saved after levelling in the fjords, and reloaded an earlier save when I finished off as much as I could (I made it past level 22, but there was no way to get to level 23, so I reloaded and turned in just enough quests to reach level 22, so the mindread got me to level 23). You don't need to get that high a level; just save after levelling and go through the fjords until you either start to have trouble or decide you just want to head to the island.
