Learn Portuguese (Portugal): A caravana cruzou a vastidão do deserto durante a noite. - The caravan crossed the vastness of the desert during the night.

You would place não before the verb and senão before the restricted object: A caravana não cruzou senão a vastidão do deserto durante a noite. This emphasizes that they crossed absolutely nothing else, matching the restrictive negation pattern you are currently learning.

The word vastidão ends in -idão. Is this a common pattern for making nouns?

Yes, the suffix -idão is often attached to adjectives to create feminine abstract nouns expressing a state or quality, similar to '-ness' in English. Here, vasto (vast) becomes vastidão (vastness). Other common B2-level examples include solidão (loneliness, from só) and gratidão (gratitude, from grato).

Since crossing the desert took an entire duration of time (durante a noite), why is the verb in the Pretérito Perfeito (cruzou) instead of the Imperfeito (cruzava)?

We use the Pretérito Perfeito (cruzou) because the crossing is viewed as a single, completed event that was successfully finished. Even though it took time, the action has a clear boundary. If you used the Imperfeito (cruzava), it would sound like an incomplete background action, such as 'While the caravan was crossing the desert, something else happened.'

Could I say or instead of ?