A group of people get trapped in an unknown construction. Uninhabited. Nobody takes care of the place. There are no living creatures around.
They were prepared to go inside and spend some time there, but not on long term. They have some equipment - sleeping bags, some food, some water. But they were prepared to go in for a couple of days maximum, and they definitely weren't prepared for such a long journey.
They cannot find the exit for three weeks.
Desciption of the place: This is a construction by an alien sentient race that does not know humans. It's a vast closed place with no exit and no food lying around. They get lost and spend about three weeks time wandering.
It's a alien projection of what a human lodging might look like. It includes artificial and natural elements, as a mish-mash of wooden corridors, cave tunnels, and passages that don't resemble much here on Earth.
There is magic in this particular construction. The world around isn't exactly magical (at least not the type of magic humans can use), so the characters are unfamiliar with the setting, but in the construction weird things can definitely happen. This is how they can't find an exit for three weeks: the corridors keep changing and making wormholes, so it's impossible for them to trace back.
The reason why there is no food lying around is because it is not built for humans. Instead, the aliens built a model, sort of a decoration, so they wouldn't put human food in there on purpose. Even if they wanted to, the aliens don't have a concept of what a human needs to survive. It is meant to imitate the elements you could find around humans (such as structures of buildings and natural structures), but there wouldn't be details you find in humans' daily lives.
There could be sparse water sources or other ways for the humans to get water, for example: open roofs, ponds, magic streams, etc...
Outside the construct, there isn't anything extraordinary. It's standing on a grass plain not far from a large city.
Whatever is inside the castle might not make sense to the humans, even though they would be able to describe it. For example, a brick corridor might lead into an underground tunnel that leads back to the same corridor. It's not typical for human architecture, but it's possible to imagine and retell.
Space is weird. You cannot retrace your steps, because whatever is behind you changes.
The basic architecture is passages and halls and rooms and open spaces, but all the places are mostly bare. No decoration inside, no objects.
Basic setting elements would include: wood, brick, ground, rock, glass, water. they can be arranged in a number of ways to make it seem like the passages change, even though in the end the journey does become monotone.
Other than that, modifications might apply.
Question: Given that there is no evident source of food, what element could be found in such a place that would either nourish a group of humans, or give them an alternative source of energy?
(So as not to make the question too broad, assume that water is not an issue).