At the bottom of a well in a plague-stricken village, there might be a massive hole leading to the Dark Territory. A gang's underground hideout might also have an elevator leading there, and the Green Hag's lair may contain a teleportation array connecting to it… there truly are entrances to the Dark Territory scattered all over the world.
Of course, while there are many such entrances, most only allow a single adventurer party to traverse using climbing tools or spells. You can't bring pack animals through, nor can large forces pass. For raiding parties or trade caravans, these entrances have little value and are hardly worth paying attention to.
Leon had thought that Cecilia's search party would take at least three days to locate a cave leading to the Dark Territory. Perhaps one hidden deep within a forest, or maybe along some coastal cavern. To his surprise, she found one the very next day.