Hand to hand combat with rats -- a true story

I forgot to mention that three smallish rats somehow got into the apartment over the past year and a half. I hate violence in all forms but it was them or me --- and they are vicious, so what could I do? Since I don't believe in chemicals, I finished off one of the creatures with a copy of a famous work of borrowed experimental fiction (a thin one, not a fat one like Infinite Jest hardcover, which would have been easier to use.) The question is, how did three rats get inside the apartment in the first place? It makes me wonder if the rats were there all along, just biding their time, or if they were living in the walls and somehow got out.