Read the excerpt from "The Red-Headed League." Then Wilson told us that his assistant spent a lot of time in the cellar. When I saw that there was a bank right behind the store, I thought immediately of a tunnel. When Spaulding opened the door to the store with dirt on his knees, I knew I was right. Spaulding and Ross were digging a tunnel from the pawnbroker’s house to the bank.

The word cellar most likely means

“an attic.”
“a room.”
“a hallway.”
“a basement.”