Ancient sites are located where they are for a range of interlocking reasons. What exactly caused their builders to place them where they are is not exactly known, though here are the known factors that clearly seem to influence it.
The placing of ancient sites
Several factors affect the location and design of ancient sites.
Blind springs (called water domes in USA) are up-welling underground water-flows which hit an impervious rock layer and then spread out more or less horizontally, setting up radial and spiral energy-patterns on the surface of the earth that have been shown to be a big factor in the location particularly of stone circles. Sites such as standing stones and cairns are often located at the crossing points of underground water lines - subterranean water streams and trickles moving through fissures and porous areas of rock below. This is what dowsers pick up when they're examining megalithic sites in the field. More about this here.
The visible landscape and intervisibility
The Britons of ancient times had their own form of feng-shui or landscape architecture. For example, both Tregeseal and Nine Maidens stone circles are overshadowed by prominent hills – Carn Kenidjak and Carn Galva respectively - forming part of their scenic backdrop. There was a certain landscape artistry to this, the building of a poetic and mythic psycho-geographical landscape, stuffed with character, history and hidden significances.
This landscape factor involves intervisibility too - seeing other sites from one site. This visible connection with other sites is important at some, but not all, sites. Some sites are on top of a hill (such as Bartinney Castle, Botrea Barrows or the cairns on Chapel Carn Brea) while others might be carefully located on the side of a hill so that, when looked at from another site or from a particular location, they stand out on the horizon. This is the case for the menhirs at Boswens, as seen rom Tregeseal, and on top of Watch Croft, Penwith's highest hill, as seen from the Nine Maidens stone circle. If you stand at a site like Mulfra Quoit and swing around, you'll see an array of different sites and hills, some of them quite remarkably placed in relation to Mulfra.
Sometimes, when approaching an ancient site, one wonders why it is located where it is. Then, when you arrive at the site a vista and visual context appear that suddenly makes sense, specifically at that precise location. If you move a short distance away this panorama or perspective disappears. So this siting business is at times really carefully calculated. Just above Carn Lês Boel, on the landward side, there is a small cairn which you can stand on, and Carn Kenidjack, some kilometers away, becomes visible - but move just a few yards away and it's gone.
The rising and setting points of the sun at the solstices and other key times of year such as the cross-quarter days, and those of the moon at its major and minor standstills, are important factors in the design and location of ancient sites. It probably applies also to the planets, but they follow more or less the same course as the Sun across our heavens, so it is impossible to tell whether their paths were specifically built into ancient sites.
This applies particularly to sites such as stone circles and certain standing stones or chambered cairns. Some sites were archaeo-astronomically located to optimise viewing of the rising of heavenly bodies. For example, from Chûn Quoit, the summer solstice setting sun sets in a nick in the summit of Carn Kenidjack, a kilometre away. From Trencrom Hill, the summer solstice sun rises over St Agnes Beacon, up the coast, and sets in a nick between Trink and Rosewall Hills northwestwards, exactly following the incline of Trink Hill until it sets.
Trencrom Hill behind, Gull Rock in front. Taken from Porthtowan
The mythic and cultural landscape
Every landscape feature had its own narratives, history and beliefs attached. These beliefs and mythologies often have some relevance to the nature of the site. Some prominent hills such as Carn Galva, Trencrom Hill or St Michael's Mount were regarded as the homes of giants, for which traditional tales of their antics still exist today.
Our perception of an area is still today influenced by traditions, stories and associations - about someone who lived there or something that happened - and this was very much the case for ancient people. Theirs was a magical landscape infused with presences and significances, and this wasn't just the fantasy of primitive people - it was a magical perception with substance to it, that we need to reclaim now, to heal ourselves of a tendency to exploit and destroy nature and exorcise the landscape of its spirits. It's a loveless condition that sees nature as a resource, a threat, property or an object with no emotional or imaginal content.
Ancient sites, with their special atmospheres, attract to themselves associations, mythologies, events and experiences both actual and numinous. Places have identity, and 'strong' places have characteristic atmospheres and resident presences - genius loci, the spirit of place.
These are very important, and a major theme on this website. The system of backbone alignments in particular is anchored in natural features such as hilltops and headlands, influencing the location of major sites such as stone circles - so the site was built in alignment with pre-existing natural sites. These are focal sites. Subsequent or subsidiary sites will be built in alignment with these - they are more derivative sites, at least in terms of alignment. More about alignments here.
Mathematics and geometry
In other parts of Britain abstract patterns and shapes have emerged in the way that ancient sites are located, and this is probably true in West Penwith too. Apart from a few such patterns (see here) this question has not yet been properly studied in West Penwith.
Bundles of purposes
Many ancient sites are located where they are for a number of combined reasons, and what's remarkable is that this can be so. Sometimes it is not immediately visually obvious why a site is where it is - Chûn Quoit, Carfury menhir or even the Merry Maidens are examples. Why are they on the sides of hills? Other sites are blindingly obvious - Lesingey Round, Chûn Castle or Castle an Dinas being examples.
But somehow, the above-mentioned factors knit together into a wholeness that determines the location and patterning of ancient sites. Every site has a uniqueness to it, yet in an organic and intuitive way it follows similar rules to other sites. Usually a site exists not for one single neat reason.
When you visit them, it's good to go quiet, imbibing the feeling of a place, listening, hearing and seeing what it has to teach. What you 'get' isn't 'just imagination' - you can pick up on something from the place-memory or innate identity of the site, its genius loci.
The purpose of ancient sites
Mounds (cairns, barrows and tumuli) sometimes show signs of use for burial of the dead, but this does not mean they were built for burial. Later in history, churches were surrounded with graveyards, though they were built to be places of worship and community gathering, not primarily to host the dead. Both mounds and churches were built for a number of interlocking purposes, of which burying the dead was one, and a secondary purpose.
Unfortunately, archaeologists tend to place undue weight on ancient burial practices, thereby missing other, more important, reasons why mounds were built. They assume that since some mounds had burials and bones, all must serve a funerary purpose. Not true.
In the 1960s-70s Guy Underwood and Tom Graves, both dowsers, investigated this, coming up with the observation that mounds acted as energy batteries for the accumulation and release of energy, or as flow-smoothing energy or information buffers, playing a part in a larger geomantic system. Standing stones, meanwhile, served more as conductors or antennae connecting earth and heavens - acupuncture needles in the land. From an energy-engineering viewpoint, this makes these sites far more plausible and meaningful.
It would be wonderful to be able to read the minds of neolithic and bronze age people, to understand what was in their thoughts when building such a large number of sites as we see in Penwith, but we cannot - we can only see the signs and remains of what they did. Here plausible speculation, intuition and imagination play as much of a role in understanding ancient sites as concrete archaeological evidence and its interpretation - in which are often invested many assumptions and elements of guesswork. However, for such speculation to work, it does need to be supported or verified by other considerations that would give more strength to the speculation.
There can be a tendency to impose on ancient sites ideas that come from modern times, when in fact the ancients of 4-5,000 years ago might have had entirely different ideas. One of these modern ideas is the notion that subtle energy does not and cannot exist. We moderns largely don't perceive it, therefore it isn't there. But this is a time-limited belief.