It's been a long time since I played, but I vaguely recall that once you're done with the rebels (I assume you've been through the various rebel "backup" bases as well?), you need to go to the village and talk to someone to trigger a quest to go explore the Ziggurat on the Eastern end of the island.

As far as tips:
1. Use spells. Attack spells aren't all that useful until you start to get the more powerful ones later in the game, but you should be making regular use of healing and buff spells. Your mana regenerates pretty quickly, so you don't have to be too conservative with it. Also, just in case you didn't notice, you can customize the power of each spell in the spells menu using the blue sliders (though this also affects mana cost).
2. The game world is fairly open, so there are areas you can gain access to early on that will kick your ass. Don't worry about this - it usually just means you need to come back later.
3. Since there's no XP or loot system, dodging past clusters of enemies you don't feel ready to take on is a viable strategy.
4. I don't remember if you've gotten an of the jars of honey yet, but I recommend saving them rather than drinking them right away. Yes, they give you a permanent boost to max HP, but they also instantly refill your health (and possibly mana too, I don't remember), so I tend to save them for use as health potions.