The games are designed so that a knowledge of military tactics will facilitate good gameplay. Tactical wargames offer more of a challenge to the designer, as fewer variables or characteristics inherent in the units being simulated are directly quantifiable. Modern commercial board wargaming avoided tactical subjects for many years, but since initial attempts at the subject appeared, it has remained a favourite topic among wargamers. Perhaps the most successful board wargaming system ever designed, "Advanced Squad Leader", is set at the tactical level.