Doorways and thresholds do not exist in nature: it requires a human being to conceptualise even a cave opening as being an 'entrance' from one location to another. Animals may understand variations in temperature or light; they may understand comfort vs discomfort; ants may have a hormonal sense that they are within/without their nest. But it takes human intelligence to have a grasp on the concept of a space which itself constitutes an opening into or out of - a gap which is literally liminal. Not empty, because something is there even though it is not.
Passing through thresholds has long been understood to be an ontological act - a way of going from a humdrum reality to one in which Adventure takes place. Sometimes the thresholds are literally doorways, though they might not be in places one would expect (The Lion, the Witch and the Wardrobe). Sometimes they are of a less obvious kind: think of Alice going through the looking glass, or down the rabbit hole (Lewis Carroll was obsessed with doors, and there are of course doorways-within-doorways-within-doorways in his work). Sometimes they are hidden or magical (as in Harry Potter); sometimes they feel as though they trade on being not quite literal or metaphorical but somehow both (as in The Secret Garden). Sometimes they are imbued with mythical or religious significance - Theseus going down into the caverns of Minos - while at others they are technological (think of Neo passing into and out of the Matrix through a cable jacked into his brain). Sometimes they signify adventure through passage outwards (like Bilbo going out of his front door); sometimes they signify it through passage within (like the crossing of the threshold into Moria). Thresholds, in speculative fiction, are everywhere.
And they are also everywhere in tabletop RPGs. There is something that we sense to be important about passing from one ontological condition, the condition of there being nothing particularly at stake (the tavern, the market, the village, etc.), to another - the condition of there being everything up for grabs. And the point of passage or embarkation from one to another strikes us as significant. We like the idea of transferring from one to another and back again.
Thresholds can appear, and operate, in three ways.
First, there is the threshold of the most obvious kind - that which signifies entry into adventure. The stereotypical example is the dungeon entrance, the tunnel, the cave, the crevasse, the descent into the Underdark. Here, the PCs are most in control of their fate. They know where they are going. They know that death awaits.
Second, there is the type of threshold that signifies exit into adventure. Here, what is at stake is Out There in the big bad world, and the threshold is the city gate, the harbour, the bridge, the time machine, the transporter. The PCs start within a place where everything is understandable, manageable and graspable, and they go out through the threshold to something vast, open and chaotic. It has the feeling about if of unpredictability, much more so than going into, say, a dungeon. A feeling of an abandonment of control,
Third, and least well understood, is the type of threshold that signifies intrusion. This type of threshold troubles us. We are more comfortable with the first two types of threshold because they are volitional. The adventurer, usually, makes a choice, or at least performs a positive act, to go through. But here, in this third category of threshold, things work the other way around. The adventurer does not cross a threshold, but adventure instead passes through to look for him. Demons, goblins, evil spirits, magical entities, pass from one place of being into ours and thereby threaten it.
The third type of threshold is most closely identified with horror because it signifies the potential for ontological disruption. Just as the presence of an unwanted person in one's home - whether a burglar or an unwelcome guest - seems to make it unstable, to make it no longer feel indeed as a home at all, so intrusion from 'out there' into 'in here' changes the fundamental nature of 'here' itself. 'Here' is transformed into something altogether different. And the quality of being itself thereby shifts. From safety, security and the known into danger, hostility and mystery.
Thinking about the type of threshold that one wishes to deploy may therefore be a useful conceptual starting point when thinking about the style of campaign one would like to play and the mood one would like to facilitate. What is being crossed? How is it happening? And who is doing the crossing - the adventurers - or that which lies beyond?