Like many other solid planets, the Otherworld has multiple layers of rock and molten material inside and covering it. These layers primarily are the crust, the mantle, the vacuum, and the core.
The crust is a solid mass of soil and rock that all the living things of otherworld reside upon. It is shaped like a cylinder, and covers all of otherworld. Occasional cracks in the crust result in anomalies such as the volcanoes of Alvalpa. The crust is kept in place by giant pillars of iron that attach it to the middle mantle.
The mantle is divided into three layers within itself: the upper mantle, which is a massive area of molten rock right below the crust, the middle mantle, which is composed of radioactive materials and is attached to the crust, and the lower mantle, which, along with the core, helps keep the vacuum in place and is composed of extremely dense rock.
Underneath the mantle is the vacuum. This large expanse has absolutely nothing in it. Instead of sucking Otherworld in upon itself in order to fill this gap, the vacuum is kept from destroying Otherworld by being secured between the lower mantle and the core. To the naked eye, it would appear that the core floats inside the vacuum.
The final layer is known as the core. It is a relatively light cylinder of tin at the center of Otherworld. It serves as the thing that keeps Otherworld in orbit, as without it, Otherworld would drift aimlessly across space. A strange, seemingly impossible physical anomaly is the reason that the core does this. You see, the other layers of otherworld formed around the core. If you make a stack of books, and take the bottom one out, the rest of the stack will fall over. This effect also applies to the core of otherworld. Though it seems impossible, astronomers and mathematicians have repeated the same formula again and again and have always come up with this same result. It is known as "The Pylon Effect" by scientists. The core is locked in orbit, just like the bottom of a stack of books keeps the others from falling down.