As Stabbey said, if there is still a barrier blocking access to the path leading up to the battle tower (on Sentinel Island), you need to use the book of magic (Arcane Notebook) that can control wind, received when you rescued the guy on the top of the pillar (near the trainer Kenneth). The windy spot, around a tombstone and three candles, is near where you start on the island.
Speaking of the windy peninsula, have you done any swimming around the island?
Jump into the water to either side of that peninsula, and you can swim to the entrance of a hidden cave.
You can get most of the way through the battle tower before you have to worry about finishing off everything in Broken Valley. There is a warning in-game before that point (once you enter the summoning chamber there is a fight before you confront Laiken and take over the battle tower, after which most quests and NPCs in Broken Valley and on Sentinel Island will no longer be available).
