Learn Finnish: Ryömi yläkertaan hiljaa!

This sentence demonstrates the "movement to" part of the directional paradigm. In English, "upstairs" acts as an adverb, but in Finnish, yläkerta is a noun (literally "upper floor"). Because the action (ryömi) involves movement into that destination, it requires the illative case ending -an. If you were already crawling around up there, you would use the "location in" case (yläkerrassa).

Why does yläkertaan keep the strong -rt- instead of changing to -rr- like in yläkerrassa?

Consonant gradation from strong rt to weak rr happens when a suffix closes a syllable. The illative ending -an lengthens the vowel, and the syllable break falls before it (y-lä-ker-taan), leaving the syllable open. Because the syllable remains open, the strong grade rt is kept.

Is ryömi in the past tense here?

No, it is the singular imperative (command) form of the verb ryömiä (to crawl). You form it by taking the present tense stem (ryömi-). While it looks exactly like the third-person past tense ("he/she crawled"), the context as a direct command without a subject pronoun makes its meaning clear.

If hiljainen means "quiet", why is the word for "quietly" hiljaa instead of hiljaisesti?