A ruby mine. When rubies are ready to be collected, a mine cart full of rubies comes up to the end of the tracks in front of the door to the tower.

The ruby mine is a small, cone-tipped tower made of stone with a blue cone-shaped top and mine cart tracks running down into a tunnel next to the tower. The ruby mine is a once-in-a-lifetime special offer presented to low level players (below level 25). The offer is presented as one of the events below the castle, with the name "Ruby Prospector". The Ruby Prospector claims to have discovered a large amount of rubies below your castle, and offers to mine them for a one-time payment of 30000 rubies. He will give 250 rubies every day for 360 days, totaling 90000 rubies, which is three times the original price. Of course, subtracting the original 30000 rubies paid, the profit is 60000 rubies. Essentially, it's a 200% bonus on 30000 rubies dragged out over a year.