Defensive Perimeter

You create a 30x30 square on a moderately flat surface. 5 foot high wooden-spike walls sprout out of the ground around the area. All creatures trying to get past the walls must make a DC 15 Dexterity check or a DC 20 Strength check. If failed, the creature takes 3d6 piercing damage. All creatures inside the square have half cover from the sides and One-quarter cover from above. Any long rest taken inside the perimeter of the square Gives all spellcasters one extra 1st level spell slot, that functions similar to temporary hit points. All attack rolls coming from inside the wall towards a target on the outside have advantage.