No sooner__________________(我们到达山顶) than we all sat down to rest.

The sentence requires inversion due to the negative adverb "No sooner" at the beginning. In such structures, we use "had + subject + past participle" to form the past perfect tense before "than" introduces the subsequent action.

"had we reached the top of the mountain"

This construction emphasizes the immediacy of the second action (sitting down to rest) following the first (reaching the summit). The full sentence reads: No sooner had we reached the top of the mountain than we all sat down to rest. This pattern is commonly used in formal English to highlight the close timing between two past events.