A sentence that has two or more main clauses and one or more subordinate clauses is called a compound-complex sentence. For example, Jessie reads novels, and Lucas reads poetry, but Gordon reads magazines because novels and poetry are difficult to comprehend. "Jessie reads novels", "Lucas reads poetry", and "Gordon reads magazines" are the independent clauses, while "because novels and poetry are difficult to comprehend" is the dependent clause.
The servants do not "abscond" because Montresor tells them to stay at home; they do so because he tells them he will be gone all night. They don't care if he finds out that they all left to join in the carnival. He can't fire them because he can't get anybody to replace them.
The clause <em>when we can rest </em>is a subordinate/dependent clause, meaning that it cannot stand on its own - it has to be a part of a larger, independent clause, as is the case here.