Questions & Answers about Innerhalb des Verstecks muss es sehr kalt sein.
How is muss being used in this sentence? I thought it meant "have to".
You are right that müssen usually means an obligation. But here, it is being used subjectively to express a strong assumption or deduction. Just like in English, saying "It must be cold" means you are highly confident that it is cold based on the evidence, rather than stating a rule.
Why is it des Verstecks and not dem Versteck?
The word innerhalb (inside/within) is a preposition that requires the genitive case. Since Versteck is a neuter noun (das Versteck), the article changes to des and we add an -s to the end of the noun, making it des Verstecks.
Could I use drinnen instead of innerhalb here?
Not in this exact structure. Innerhalb is a preposition, meaning it needs an object to follow it (inside of the hideout). Drinnen is an adverb that stands alone, meaning "indoors" or "inside" in general. You would use drinnen if you didn't specifically name the hideout, for example: Drinnen muss es sehr kalt sein. (It must be very cold inside.)
Why does the verb muss come before the subject es?