He found that to hold it open wide he would have to get behind it and shut himself up between it and the stairs.

Most likely, all hands being required for securing the victim, the captors would have taken the precaution to prop the door open by some means, so as to be ready for their deep-laid and carefully prepared scheme.
So Arthur groped about and discovered a twisted-up wedge of paper, which, by its battered look and peculiar shape, had evidently been stuck at some time under the door to keep it from closing-to.

He quietly pocketed this prize, on the chance of its being useful, and after possessing himself of the sack and cord, and two wax vestas lying on the floor, one of which had been lit and the other had not, he prepared to quit the scene.

As he was going up-stairs he caught sight of one other object--not, however, on the floor, but on the ledge of the cornice above the door.

This was a match-box of the kind usually sold by street arabs for a halfpenny.