Instructions
Click on the tree toward the north of the map. It's next to a fence. Select the "Demolish" icon to blow it up. After the explosion it will fall onto a button, opening the gate to the bridge.
Pick up the note that's just beyond the bridge. It will give you a hint about buried treasure, initiating the "Sunken Treasure" quest.
Select a character with good construction abilities. Click on the "Build" icon. Choose "Well." Place the well in the middle of the blue stones on the far east side of the map. Wait for the builder to finish constructing the well.
Drag and drop your town hero onto the well. He'll jump in and retrieve the treasure.