Siege of the Bone Children
by Erik Lynd
When his son dies in a terrible accident, Malcolm retreats behind the walls of the fort his son built, but those walls were more than a child's game, they served a purpose. When monsters begin showing up he realizes that when you hide behind walls to keep out the bad, you must always leave a door for the good.
This story, along with others, is collected in The Long Fall Into Midnight Vol. 1