Speaking from a MP point of view:

Turtling is a tactic often used by new players as seems easy to employ - you just sit there. Back when I played a lot of RtW MP, I almost always easily defeated turtlers. The reason for that is that by turtling, they relinguish the initiative and react to your moves, never forcing your hand. That makes them easy to take apart using archer superiority, artillery, surrounding them, etc.

Note that a good player turtling will be immensely more difficult to defeat, since they will have the presence of mind and the skill to counter most of these tactics while maintaining a good defense. Ironically, however, good players very, very rarely turtle because it's a disadvantage against a skilled opponent, and an unskilled one they don't need to turtle against.