Using a level, check the fort horizontally from the bottom of the playset to the top. Start by verifying that the baseboards are level. If you have high ground, dig it out. Move higher now, checking every tier for levelness. When checking the floor, ensure that the center of the floor is as level as the outside edges and the four corners. Check to ensure that tarp assembly is level. Check the swing beam/monkey bar brace, the rungs on the deck ladder, and the rungs on the rock wall. Check to see that the rock-wall brackets are level. Check to see that the fort angle brace brackets are level with each other.
Once you've checked the horizontal levelness, it's time to check it vertically. Check all uprights, wall slats, tarp uprights, deck ladder rails, rock-wall rails, and verify that they are straight. Once the fort is horizontally and vertically level, tighten the hardware to ensure it stays that way.
For the monkey bars/swing beam portion:
Check to see that the ground beam is level. If it is not, dig out the high ground. Check that the two green plastic angle brace brackets are even with each other. Check the rungs and beams of the monkey bars for levelness. On occasion, the monkey bar or swing beam will lean into the fort causing the assembly to become unlevel. Once it is level, tighten your hardware down uniformly and make sure it is staked to the ground.