Well done for identifying this excellent example of an implied metaphor, where the movements of Simon Wheeler are clearly being compared to some form of wall or large object that prevents escape. Let us have a look at the passage in context to fully appreciate the humour of the situation:
Simon Wheeler backed me into a corner and blockaded me there with his chair and then sat me down and reeled off the monotonous narrative which follows this paragraph.
We can see that the implied metaphor uses exaggeration for humorous effect, as Simon Wheeler seems to be compared to something like a tank or a military unit that blockades the narrator in to the corner so he cannot get away. Then we have an example of irony when he describes the tale he is told as a "monotonous narrative," as it is highly unlikely that Twain would be telling us this story if it was boring. The humour of course is developed through the course of the story and then when Simon Wheeler is called away and the narrator makes a quick getaway, not wanting to be penned in again!