Complex sentence: When invaders reached the outskirts of Rome, they were repelled by an improvised army assembled by the Senate, which consisted of local troops (probably prǣtorian guards) and the strongest of the civilian population.
Split into simpler sentences: When invaders reached the outskirts of Rome, they were repelled by an improvised army assembled by the Senate. That army consisted of local troops (probably praetorian guards) and the strongest of the civilian population.