Here, ascendere is used instransitively (Festus isn't talking about climbing the room itself but climbing up to the room). Moreover, it seems that the Romans didn't typically think of stairs as things that were themselves climbed but as the means of climbing up to something else (at least when they used the verb ascendere).
Therefore, if you want to turn the verb passive (for example, to deemphasize or entirely omit mention of the specific agent of the action [that is, the climber]), this can't be done by using a regular 'personal' passive, where quae (= cenacula) or scalae is the subject. (Note, however, that this can be done in English, where we could say 'the room that is climbed up to' or 'the room that stairs are climbed to.')
Instead, to get a passive idea in Latin by using the verb ascendere, you have to use an 'impersonal' passive.
From Gildersleeve & Lodge, Latin grammar, §208:
208. IMPERSONAL VERBS.—Impersonal Verbs are verbs in which the agent is regularly implied in the action, the subject in the predicate, so that the person is not expressed.
2. The passive of intransitive verbs is often used impersonally... The subject is contained in the verb itself.
Literally, then, the passage from Festus means 'to which the act of climbing is performed by way of stairs' or '...to which there is climbing by way of stairs'; or, a bit more loosely, 'to which one climbs by way of stairs'/'that one climbs up to by way of stairs.'
In English, we can translate this more loosely still as '...that is climbed up to,' as if the subject were the relative pronoun quae and the verb were ascenduntur, or '...that stairs are climbed to,' as if the subject were scalae and the verb were again ascenduntur.