So just reading all the explanations here again convinces me that the ending is stupid and illogical and tacked on :). Just watch yourself trying to explain it away using reasons like: "So years go by and you carve Anna's name onto your hand as a reminder, then promptly forget. Robert Lutece has second thoughts about all this multiverse shit, and makes his sister do something about it." Oh, and what about all these towers and all the other pairs visible watching out of them? From that moment on the mechanics of the world change which is why I feel it is tacked on!