Luttwak believes that `the Roman achievement in the realm of grand strategy remains entirely unsurpassed and even two millennia of technological innovation have not invalidated its lessons.' He argues that the strength of the Roman Empire was not due to tactical superiority on the battlefield, superior generalship or more advanced weapons technology. The superiority of the empire, and it was vast, was more subtle: the complex of ideas and traditions which informed the organisation of Roman military force and harnessed the armed power of the empire to political purposes - i.e. what can be called the grand strategy of the Roman empire. (The psychological aspects of this strategy explain, for example, the apparently irrational use of force in the seige of Masada in AD 70-73, involving building a huge ramp reaching the full height of the mountain.) Luttwak identifies three distinct systems of security over the imperial period, which each integrated diplomacy, military force, road networ ks and fortifications. Each system was designed to meet different priorities: first, hegemonic expansion, then territorial security, and finally, in the great crisis of the third century, sheer survival of the imperial power.