Basically, it doesn't matter what you do with science, religion or culture if you cannot protect it. Which leads to players creating sizeable armies. Also, in MP, it's just human nature to always expect the worst of your fellow players, lol. Your mind set is geared towards the military side of things. It's a must to build up your army, more so than in single player. And then you're always just one step away from building one or two more military units instead of that district improvement... and then putting them to use.