Final answer:
The sentence in question is a complex sentence as it includes one main clause and one subordinate clause connected by the conjunction 'nevertheless'.
Step-by-step explanation:
The sentence 'Nevertheless, in this mansion of gloom I now proposed to myself a sojourn of some weeks' is a complex sentence. It contains one main clause and one subordinate clause. The main clause is 'I now proposed to myself a sojourn of some weeks', and the subordinate clause, introduced by the conjunction 'nevertheless', adds additional information but cannot stand on its own as a complete sentence.