There is a door in the wall nearby, and a long wide ramp that clearly moves on beyond this immediate area. So you check the door, Nate clears it of traps, and you open it wide and rush in finding an empty guard chamber. This chamber was likely a barracks long ago, judging by the rotting bunks, chairs, chests, and weapon and armor racks. Someone has culled the worst of the furniture and tossed it toward the northern wall, while it looks like the remaining pieces have seen recent use.
You suspect this is where the Thrulls were stationed, but now, after a quick search it turns up nothing of interest. So you move the wide tunnel.
Following Poltur’s trail off the Silver Reign, you make you way down to the dry channel of the Lower Canal which connects the three islands beneath the waves. Once you reach the bottom, the lower lock door is visible (this lock is between the Lion and the Dragon and is not shown on either island’s map). From the bottom of the steep stairs, a channel roughly 20 feet wide and 50 feet high carved through the stone runs nearly straight off into the darkness, and a narrow ledge runs alongside the channel at a height of 15 feet. Sediment covers the channel’s floor, as if a river ran through it many years ago. A gigantic set of wooden double doors, 50 feet in height and 40 feet wide, interrupt the back side of the wall, forming the far end of High Port at its base. They are otherwise similar to the previous set of doors.
Water filled the Lower Canal to a depth of 15 feet when the port was operating. Lowering the water level in High Port and opening the doors when the two water levels were equal made it possible to move a ship from High Port to the Lower Canal. Valuables were often lowered by rope from High Port, and skiffs ferried sailors and goods from the north end of the Lower Canal to the many storage areas and barracks under the Dragon.
You avoid taking the long tunnel that ends with the impassible doors, because Nate picks up Poltur's trail, a trail that follows a ledge that skirts the tunnel and takes you up over the lock and beyond.