Learn German: Der Wärter träumt nie von der Stadt, sofern der Dienst ruhig ist. - The keeper never dreams of the city, provided that the duty is quiet.

sofern is a subordinating conjunction that means "provided that" or "as long as". It acts like a stricter, slightly more formal version of wenn (if). While wenn sets a general condition, sofern emphasizes that the first part of the sentence is entirely dependent on that specific condition being met.

Our current lesson is about clauses like so schwer es auch ist (however hard it is). Is sofern part of that same grammar pattern?

No, they are different types of clauses, even though both involve the word so. The structure so [adjective/adverb] ... auch is concessive—it means something happens despite a condition (e.g., "however quiet it is"). sofern is just a single-word conditional conjunction meaning provided that. However, both structures share the same word order rule: they send the conjugated verb (like ist) to the very end of the clause.

Why is the feminine noun Stadt written as der Stadt here?

This is a quick refresher on prepositional verbs! The verb träumen (to dream) requires the preposition von to say what you are dreaming about. Because von is a preposition that always triggers the dative case, the feminine article die must change to the dative der.