Learn French: Nous sommes remontés au dernier étage.

Because it is being used intransitively (without a direct object). When verbs of motion like monter and its derivatives like remonter describe movement to a place, they use être. If you were bringing a direct object back up, like 'we brought the bags back up' (Nous avons remonté les sacs), you would use avoir.

Why is there an 's' on the end of remontés?

When a verb takes être in the Passé Composé, its past participle must agree with the subject. Since nous is plural, we add an s. If the 'we' consisted entirely of women, it would be spelled remontées.

Why does dernier come before étage instead of after?

The meaning of dernier changes depending on its position. When placed before the noun, it means 'last in a physical series' or 'final' (like the highest floor of a building). When placed after a noun, it means the 'previous' or 'just passed' one, which is mostly used for time (like la semaine dernière for 'last week').