Yes, I'm not surprised that you found the PC opponent tough on easy levels. It takes a little while to get it sorted and the computer most likely is still using its strong opening book, so that you will be starting out on the wrong foot.

Don't take the "rules" of chess openings too literally. They are guides and eventually you will understand when to ignore them and when to obey them. One of the things the Ruy Lopez does is to force your opponent to respond to your aggressive bishop move. Pinning a piece to the king is aggressive and no idle threat. When black moves the pawn he has also "lost a move" and at the same time altered his pawn structure in a way that creates a hole that might later be exploited, so you aren't really violating the guideline either, but you are forcing him to commit.

The guideline is to discourage you from taking a tour with a piece, a common weakness in beginners (as we all once were.) The idea is to be efficient with your opening moves and not to "waste" any. If your opponent is staying in reactive mode *without* gaining an advantage, then the move is not wasted. I was never a Ruy Lopez player and I don't have any of my opening books with me at the moment. I usually played queen's pawn opening as white as my style matured. However, king's pawn openings are a good way to start, since they tend to be more straightforward in their logic (except when your opponent counters with something like the sicilian defense.) As black I usually played the sicilian defense vs. King's pawn and tried different variants as my style matured. I disliked locked symmetrical centers. (The computer's book will be very, very deep in the Sicilian--and my opening books were deeper, sometimes taking me well over 20 moves/40 plies deep before we left the book in correspondence play.)