Learn Russian: Это чужая территория, сюда нельзя заходить. - This is someone else's territory, it is forbidden to enter here.

This perfectly demonstrates how нельзя changes meaning depending on the verb aspect. Нельзя + imperfective (заходить) means an action is forbidden or not allowed. If the sentence used the perfective зайти, it would mean it is physically impossible to enter (for example, if the only gate was welded shut). Since this is about a rule on private property, the imperfective is required.

Why does the sentence use сюда instead of здесь for "here"?

Здесь is used for location (answering "where at?"). Сюда is used for direction or destination (answering "to where?"). Because заходить is a verb of motion that describes going into a place, you have to use the directional word сюда.

Is чужая related to the word for "foreign"?

Yes! The adjective чужой broadly means "alien," "foreign," or "belonging to someone else." In this context, чужая территория means "someone else's territory" (like private property). You will also hear it used to describe "other people's things" (чужие вещи) or as a noun meaning "a stranger."

What does the prefix за- add to the verb of motion ходить?