While Faerie Land might sound like somewhere out of your favorite childhood picture book, Spenser's allegorical twist on a classic imaginary world makes the land of Faerie much more mysterious and elusive than your average fantasy story. Unlike the worlds of say, Lord of the Rings or Game of Thrones, which have intricate, complex, and deeply embedded invented histories, territories, and communities, Faerie Land is a much hazier, less fleshed-out imaginary space. Part of the reason for this is Spenser's allegorical method, which can make all the places in Faerie seem to be either secondary to allegorical action or extensions of a character's allegorical function. For example, the cave of Mammon is an extension of his role as the embodiment of greed, and seems to only really be a place as long as he is active. In other words, the setting can sometimes feel incidental to the action itself.
What's also tricky about getting a handle on Faerie Land is the difficulty in imagining it as an actual geographical and physical place. We never hear about where different landmarks or castles are in relation to one another and between every book, the geography of Faerie seems to drastically alter such that no knight ever seems to come across the same castle, ocean, or island as another knight in a different book.
Faerie at once seems massive, since there are an endless number of castles, kings, and giants but also feels almost claustrophobically small, since within each book, characters seems to run into one other with amazing consistency. The only place that is constant in Faerie Land is the court of the Faerie Queene, a place we only hear about and never visit, so it still remains shrouded in mystery.
While Faerie's haziness may be frustrating to us who are used to encountering detailed fantastic worlds, Spenser is up to something a little bit different in constructing his imaginary world. Instead of entirely creating an alternate universe with laws, political groups, and creatures unique to it, Spenser wants to create an imaginary space that can stand in for a large number of other places.
Sometimes he needs it to be both a representation of Britain and somewhere near it (since Arthur, we learn, is from Britain), sometimes all of Europe, sometimes somewhere totally unrealistic like the Classical underworld. If Faerie had too many of its own very specific traits, Spenser wouldn't be able to fluidly evoke this multitude of places.