Yes, the Twins are easy to circumvent but extremely well fortified.
On the opposite, Moat Cailin is hard to avoid but nobody cares about maintenance.

There are a lot of inconsistency in Westeros.
The one that bothers me the most in probably the North.
You have quite a lot of people living up there, able to feed themselves in a region were it snows even in the middle of summer.
And how can the trees grow in the land of eternal winter?
That sort of things.

But the one thing GRRM does well is the story and political patchwork.