Marc Bloch and Fernand Braudel have argued that geography determined the history of the region; other scholars emphasize the absence of central political structures. The very mountainous nature of Italy's landscape was a barrier to effective inter-city communication. The Po plain, however, was an exception: it was the only large contiguous area, and most city states that fell to invasion were located there. Those that survived the longest were in the more rugged regions, such as Florence or Venice, which was protected by its lagoon. The rugged terrain of the Alps prevented the Holy Roman Emperors or various German princes and lords from attacking the northern part of Italy, safeguarding the country from permanent German political control. Largely for these reasons, no strong monarchies emerged as they did in the rest of Europe: authority of the Holy Roman Empire over northern Italian territory, especially after the year 1177, was "de facto" only nominal; instead there emerged the autonomous (sometimes "de facto" independent) city-states.