Sun Keep (Origin of)
How does the Sun Keep maintain a fixed position in the crust? What is its source of light, which shines eternally? The Sun Keep also does not erode with geological change. How is this accomplished?
Keep in mind while reading these theories that the Sun Stone was apparently powered up long before 65000000 B.C., as it was found and used extensively by Zeal in 12000 B.C., while the time travelers cannot use it until exactly 65002300 years of charging have passed. That it is absent from the Sun Keep also suggests that it was powered up long before prehistory, and simply wasn't found until the founding fathers of Zeal located it.
Light Source Only
The world of the Chrono series, as depicted in the ending, is shown not to have a very tilted axis. Conceivably, a structure at the North Pole could receive constant exposure of sunlight. This theory only attempts to explain the presence of light; the theories below attempt to describe origin, and either use this theory or attribute the light to magic design.
The Reptites possibly created the Sun Keep. They were fairly advanced, although we don't know much about their technology or understanding of magic, if any. They were able to predict Lavos' fall, however, and Azala could lift boulders with telekinesis. The real question is whether the Reptites used the Sun Stone for power. Some evidence may be inferred from Chrono Cross, in which the Reptite Dimension appears. In that dimension's history, the Reptites lived in close harmony with the planet, utilizing its elemental powers. Back in Zeal, the Sun Stone was identified as a source of elemental, planetary power; it was retired after the Mammon Machine was discovered and the scholars of Zeal decided that the society did not need the energy of a "tired, old planet." The Sun Keep thusly may have been a Reptite experiment, with the Sun Stone being placed in the keep before the Tyrano Lair was destroyed. However, this fails to account for why no Sun Stone (except the one Crono places in the Keep) exists in the Sun Keep for the millions of years after 65000000 B.C. The absence of the Sun Stone in the keep is a fundamental problem with attributing its creation to a known civilization. This may be a developer oversight in the plot.
The question remains how the Reptites were able to anchor the Sun Keep to resist changes in the planet's crust. This all hinges on their level of technology; since they later came to specialize in a symbiotic relationship with the planet in the Reptite Dimension, it is not impossible for them to have had the technology, but the chances are very slim. After all, if they were in possession of these advanced skills, they could have dispatched the Early Humans with ease.
This theory is supported by a note by Lucca, who states that
Lucca: Solar energy was used aeons ago, well before Lavos Energy existed... It might be just the thing we need to defeat Lavos...
Since Lucca uses the words "before Lavos Energy existed", and not "before it was made available", it suggests that the Sun Stone was used before Lavos' came into the picture.
Planetary / Magic Connection
Aitrus, GrayLensman, ZeaLitY
In some form or fashion, the Sun Keep has a very strong connection with the planet, and naturally occurred. This bond is physically strong enough to prevent movement by the keep from continential drift; it also supplies the constant presence of light. It was noted in Chrono Cross that certain "Power Spots" exist on the planet's crust, said to be in close connection with the planet's inherent elemental power. Perhaps the Sun Keep is an extreme case of this type. The sunlight would be a case of elemental power (and keep in mind that elemental in this term refers to real magic, not Dragonian Elements). The actual structure is resistant to geological change in this manner; it may be connected to the planet's core. If someone were able to tap into the core of the earth, they would have access to a tremendous source of energy. The Sun Keep could be a geothermal power plant, with some sort of shaft leading down to the core of the earth. Resting on the shaft through the mantle, the Sun Keep would be unaffected by the pressure of shifting tectonic plates. Whatever mechanisms maintained the power plant could also preserve the appearance of the mountain. Another possibility is that magic is used to extract the power of the sun akin to the way the Mammon Machine takes Lavos's energy. Compare this theory to the one at Sun Stone (Origin of).
This theory fails to take into account the torches that exist there, suggesting the Sun Keep was built by a sentient race. It is not impossible that the Iokans or Reptites placed them there out of wonderment and worship, however.
A few stipulations discount Zeal's possible role in constructing it. Firstly, the Sun Stone would have been found by Zeal fully powered up, since Zeal obviously did not exist for 65 million years. Secondly, the Sun Keep would have had to have been built with extraordinary foresight; it would be a significant long-term investment unless the scientists of Zeal found a way to place the Sun Stone in history millions of years before 12000 B.C., a tenet with no evidence supporting it. Lastly, this theory is also troublesome when considering that the Sun Stone (and possibly other elemental, planetary artifacts) was responsible for the magic and power of Zeal from its creation until the phasing in of the Mammon Machine. Additionally, instead of placing it in the Sun Keep after its use was precluded by the Mammon machine, the Stone was instead placed in another palace created by Zeal, the Sun Palace, complete with a guardian to prevent its theft. Lastly, the Sun Keep exists in 65000000 B.C., well out of Zeal's reach. Even the Epoch was not complete by the time of Zeal's fall, precluding time travel ability.
From: Theory (Locations)