This technique is stupidly overpowered. I remember in the original civilization putting a phalanx on a hill and watching the AI suicide units into it for 2000 years until they finally killed it, at which point you put another phalanx up. Apparently a decade of this type of abuse at the hands of thousands of players was so traumatic for the AI that it now refuses to go anywhere near ancient-era units infesting local forested hills. For those who aren't familiar with this, it's simply taking your starting warrior, finding an enemy civ, declaring war, and fortifying your warrior on a forested hill directly adjacent to their city. The enemy civ will never move a settler out of their city when an enemy is adjacent. They will turtle in their capitol for thousands of years even though they have 5-6 archers sitting there who could easily take out your warrior. I don't even think they'll move a worker out to improve their land. This strategy completely takes one civ out of the game for the cost of a single warrior. When they finally do destroy the offender it's too late, because you've REX'd all of their land and now swoop in for the kill on their juicy capitol. This strategy is broken on emperor and immortal; I'm not sure how well it would work on deity with the second settler the AI starts with. I really hope Firaxis changes the AI to be able to deal with this kind of thing, as I cannot resist the temptation to use it and feel very dirty afterwards.