I am in Portugal (Lisbon) at the moment, and when leaving my room late at night (12 pm ~ 04 am) I usually face about 4 of these arthropods in the corridors:
(For reference, the "line" in the image is a standard separation on the floor. This particular example of the animal is a little smaller than a nail.) They seem to have quite variable sizes and walk primarily on ground. Also, as implicit above, they seem to be active primarily during the late night. (I don't see them much often, if at all, during the morning/afternoon.) They can be quite fast, but if left unharmed do not move much. They also appear to take shelter inside the walls.
From what I searched, it seems possible that they are centipedes. I've seen sites saying that they are useful to keep other less pleasant animals at bay. My questions are: What are those animals, and should I leave them be or not?