In my research I have come across an interesting epithet of Apollon that may very well be linked to his epithet Agyieus, god of the roads, and is part of his domestical cultic personality. Agyieus of course we recall is the god who is honored at the road, before the gates into the domestic abode. However it seems that Apollon was honored too just before the entrance of the home at the doorway prior to entering. Certainly in Sophocles’ Elektra we see reference of Orestes making prayers before the gods of his father’s household at the front door prior to entering (whereupon he enters with the task to slay his mother as Apollon’s oracle had dictated), followed immediately by Elektra’s address to Apollon at the doorway.
This Apollon is Thyraios or Thyraeus from my understanding, meaning, from the root thyra, the god who stands before the entrance. On one hand it seems to have a certain connection with Agyieus that may imply that it was the same cultic function in the household with two slightly different attributes in name, Agyieus designating as a protector of evils that may enter from the road, and Thyraeus protecting the boundary of the household entrance in general, at the other side of the door from that of the goddess of the portal (whether it be his twin Artemis, Hekate or both). It was here at the doorway that it was decorated in honor of the twin gods in regards to the birth of children and their survival, and it is hear that Apollon brings good things into the household, something ridiculed in early christian writings, such as that of Tertullian who sneared at the door worship provided to Apollon Thyraeus who guards the household.
This epithet may also have a connection to Thymbraeus of Ilion whose temple was at a boundary at which Achilles was injured by Apollon via Paris. As a god with a relationship with Trojans and in the later writings of Virgil, in character much like the Athenian Patroos which I have seen some good evidence likening him with Agyieus we see the idea of the protector god as a kind of paternal deity standing before the “household”. Thymbraeus also has a certain structural quality to the epithet that is reminscent of song and meter that we can understand perhaps that this lord of the doorway is truly the lord of voice of the heavens, the divine singer who makes divisions in nature and protects them as per natural design. If we consider Thymbraeus as the lord of the doorway to Ilium, we are given the image of a god who would protect the Trojan city from the attack of hostile outside forces as a larger echo of the more intimate household function of the god.
In any case, with the concept of the doorway, we have Apollon associated with the immediate entrance and point of passage not uncommon in his general worship. It would be of little wonder that poetic inspiration has a similar relationship in its root word, as thriasis, serving as a connection between the divine and mortal worlds. This is certainly also appropriate from the god who was nursed by the bee-like nymphs, the Thriae who hummed about Parnassos as they lingered over the general area of the navel of the world. thus associating Apollon in his mythic infancy even with the doorway between worlds.