Portico of Celtic temple of Rocquerpertuse, 3rd or 4th century BC. The three pillars have niches for skulls, and the lintel has carved horse heads on it, with a goose presiding over the whole.
Slightly odd juxtaposition of photos but the question is did trilithons have any meaning in themselves. Thinking about this Stonehenge is a stone monument made with carpenter skills. Someone or some people, had gathered round in a tribal meeting, looked at the great wooden henges, and the stone circles and said why can't we make a stone circle in imitation of a wood henge.
What they did shows great skill, but it also showed up that its much easier to carve in wood than stone, unless of course you have some soft easily carveable stone around. Stonehenge is therefore an imitation of something else, though of course it is unique in this aspect.
So why put the two together, it was the question, what lies behind the shape of a trilithon; for instance it is immediately a doorway/portal to somewhere else, but we know that Stonehenge is a circle, and that in the case of a roundhouse the space between the wooden posts would have been filled with wattle hurdling, so therefore the trilithons at Stonehenge are not doorways, its entrance coming from the Avenue side.
But the portico, the act of entering into another world, has of course been taken up by later religions in the Celtic world. A facade/doorway into the 'place for ancestors' for instance can be found on the Cotswold longbarrows 5000 years ago - West Kennet and Wayland Smithy come to mind. A doorway/entrance delineates the difference between the outside world and an inner sanctum, even though that may only be a humble domestic interior. Perhaps we should look to the materials used, a pole tent, in which skin or cloth is used, relies on a triangular network of poles to keep it upright. At Wayland Smithy the original timber mortuary house had such a ridge effect, there was even two stones balanced against each other. All this shows of course is technology being concieved, ideas going forward through time. The trilithon developed in a logical fashion, balancing a third horizontal pole on two vertical - neat and simple, joining them of course was the next stage in carpentry, a mortise and tenon, as seen on Stonehenge, is the method.
Of course there are other types of arches, gates and porticos, one at the Roman fort of Caerleon, this time a tetrapylon - a four pillared open-sided archway, which according to the literature survived into the 13th century, see below. It was here at Caerleon, in Wales that the Romans overcame the Silures, and gained a foothold in Wales. Also of course the saints Julius and Aaron were put to death here by the romans in 304 AD, perhaps they would have been marched through the archway to their martyrs death.
"Excavations in Caerleon, the headquarters of the Second Augustan Legion, have demonstrated the existence of a tetrapylon at the centre of the Roman fortress. Evidence indicates that the structure survived into the medieval period when it was undermined and demolished. A recent review of ceramic finds associated with the demolition horizon suggests that the tetrapylon was razed in the thirteenth century. While stone-robbing for reconstruction of the medieval castle in Caerleon may provide a partial explanation for the destruction, political circumstances at the time provided additional incentives."