まるで + [noun] + のようだ / みたいだ まるで + [verb (plain)] + ようだ / みたいだ / かのようだ まるで + [verb ない-form]
まるで is an adverb that sets up a comparison: "just like", "as if". It comes early in the sentence, and the sentence ends in ようだ, みたいだ or かのようだ. A noun takes の before ようだ and nothing before みたいだ.
The comparison is not literally true: the room is not a sauna. That is what まるで adds. ようだ alone can also mean "it seems", a guess from evidence; with まるで in front, it can only be a likeness.
After a verb, かのようだ underlines that the thing is not so: "as if – though in fact not". It belongs to writing and careful speech; みたいだ is the casual choice.
With a negative, まるで means "not at all", like ぜんぜん全然, and needs no ようだ.