Minecraft skeleton range?

I have built a large castle and want it to be "protected" by mobs but as soon as I enter the survival I will not be shot down by the skeletons (the wall is 10 blocks high)

How can you change that (the follow range works as much as I know only if you have seen one by running after you until you are more than (specified number) away)

It just doesn't work that way.

Is it difficult?

No it's not possible. The skeletons will never shoot you from such a high wall and they generally don't really shoot far. Try to make loopholes for the skeletons in two blocks.

The mobs may not see you because they are too high. Place them further down or in the wall.