LOCATION: From Cannock Castle, head west 17 steps, then move due south to find the portal on the map.
If you want to move forward with your quest now that you have the silver key (you do), then you'll need to head through the Western Monolith to get to the mainland where Moonbrooke Castle is situated. That's also where you'll find the final companion for your trio. Before heading that direction, though, make sure you've leveled the prince up to around level six so that he won't fall almost instantly in battle if an enemy picks on him instead of the hero.
When you enter the monolith, you'll be greeted by an old man and two guards. The old man mentions the monolith west of Midenhall, while the two guards wave you through to the staircase that leads below. Go ahead and descend the staircase.
You're now in a cavern. In that cavern, you can either head left or down. The left path leads to a monolith on a dead-end island, so you should instead follow the path leading directly down. Along the way, battle ghost mice and magicians (who can cast damaging fire spells and also have rather large life meters) until you reach the stone wall, then head left into the next chamber and climb the stairs to exit the area. Your next goal is the somewhat remote town of Hamlin.