The obstacle to my marrying Alicia was the thing that most troubled me.

If I only found out what it was, and if I made light of it (which I was resolved beforehand to do, let it be what it might), I should most probably end by overcoming her scruples, and taking her away from the ominous red-brick house in the character of my wife.

But how was I to make the all-important discovery?
Cudgeling my brains for an answer to this question, I fell at last into reasoning upon it, by a process of natural logic, something after this fashion: The mysterious top of the house is connected with the doctor, and the doctor is connected with the obstacle which has made wretchedness between Alicia and me.

If I can only get to the top of the house, I may get also to the root of the obstacle.

It is a dangerous and an uncertain experiment; but, come what may of it, I will try and find out, if human ingenuity can compass the means, what Doctor Dulcifer's occupation really is, on the other side of that iron door.
Having come to this resolution (and deriving, let me add, parenthetically, great consolation from it), the next subject of consideration was the best method of getting safely into the top regions of the house.
Picking the lock of the iron door was out of the question, from the exposed nature of the situation which that mysterious iron barrier occupied.