Okay, while they did not have time to create thousands of bows, which would have help them out a lot but to make a decent bow takes time, which they did not have.
But they did have the man power and thousands of horses.
So they could have built a series of ditches that were 10-15 feet deep at 200, 150, 100 & 50 yards out from the castle.
They also could have use the horses to drag logs back from the forest.
They could have lined the sides of the ditches closest to the castle with the branches from these trees (just like the Romans did at Alesia).
The 150 yard and 100 yard ditches should have had 20 foot tall stakes the have bundles of straw soaked in oil for make shift torches.
The 50 yard ditch should be 15-20 feet deep with a wooden wall so that the defenders have something to fight behind.
You put your heavy Infantry behind this wall and put the other Infantry on the flanks.
The artillery should be place off to each side with the Dothraki protecting them. Also instead of huge rocks/missiles they should be firing off far smaller rocks/missiles.
These would cover more ground then the large missiles.
Also if given time the space between the 100 & 50 yard ditches should have wooden barricades so that the Undead are broken up and cannot use their mass to smash into the battleline.
