The Sea of Tranquility, or "Mare Tranquillitatis," is one of the most well-known lunar maria, a vast basalt plain on the Moon's surface. It's famous for being the landing site of Apollo 11, where humans first set foot on the Moon in 1969.
The phrase "remains of the kingdom" is a bit ambiguous. If you're referring to the idea that the Sea of Tranquility holds historical or symbolic significance—perhaps akin to the remains of a once-mighty