King Jack and the Dragon by Peter Bently and Helen Oxenbury. Mighty King Jack along with his men have quite the day building up a strong castle -- "A big cardboard box, an old sheet and some sticks, a couple of trash bags, a few broken bricks..." -- and fighting off the dragons and beasts with their swords. But when Jack's men are taken away by giants at night, can he still fight the dragons alone?
Maybe even kings need a little help. Sometimes.