I don't understand how to play Inferno, especially how to defend against any decent rush on the 2nd week. :)

Start with a Pyre, on day 1, buy her a book, and go as far away from the castle as possible. The second wave (demons and pits) - clear out everything near the castle that is guarded, and the third wave (ifrits) - finish off another direction, and usually on day 17, devils are placed, sometimes earlier if the ogres are standing there from the start. Devils are placed in 33% of cases on day 1... well, who can withstand Inferno's rush on the 2nd week then? ;)