How did this cat get into our house?

Neither of my roommates actually has an idea of where this cat came from. This photo was actually taken on the last occasion (January 13th) that it just appeared in our house. There are no obviously holes in our house, and I find it hard to believe that it could just crawl into our flat like this!

But hey, cats are adorable. They purr inconsequentially, roll on their backs carelessly. Even with all these signs of ingénue charm, cats are a potent source of allergic reactions for yours truly.