Complex sentence: The Capitoline temple lay within Rome's sacred boundary (pomerium); the Aventine lay outside it, and in most versions of Romes founding myth was the hill on which the unfortunate Remus lost to his brother Romulus in a contest of augury.
Split into simpler sentences: The Capitoline temple lay within Rome's sacred boundary (pomerium); the Aventine lay outside it. In most versions of Romes founding myth, this was the hill on which the unfortunate Remus lost to his brother Romulus in a contest of augury.