Though it is expensive to turtle, it is more expensive to keep a standing army on its doorstep, to the extent that the neighboring kingdoms would only want to try a surprise attack. A siege around a city is expensive to maintain, and then can only hope to succeed because the city cannot grow its own food. Sieging a country which is self-sufficient (at a very basic level) will never actually succeed, on top of being harder to maintain, day-to-day.