Fighting pit in Horryn? (mild spoilers)

After beating father and people being in town, I found a secret door that lead to another door that had been otherwise inaccessible, which then lead to an area of the sewers that was behind a perma-locked door earlier.

The area contains a fighting pit but is otherwise; is this something I missed earlier or is there some purpose for it? I can't seem to find anyone saying anything about it.