Markers along the trail will tell you the story of Mule Hill. The account I have compiled below is based on Hampton Sides' book Blood and Thunder. The details vary a bit from what is written on the markers, but the gist is the same.
The year was 1846 and the United States was at war with Mexico. After capturing the territory of New Mexico, General Kearny and 300 dragoons from the Army of the West rode west to conquer California. En route, Kearny intercepted the famous scout Kit Carson who was carrying dispatches announcing that California had already been subdued by American forces. At this point, Kearny sent two-thirds of his force back to New Mexico to help reinforce Santa Fe and continued on with 100 dragoons as well as Carson who was recruited to guide them to California.
It wasn't until they reached California that Kearny learned that forces loyal to Mexico had risen up and retaken most of California from the Americans, leaving just a single American stronghold standing along the coast at San Diego. When Kearny's dragoons encountered a mounted Mexican force under the command of Captain Pico near the village of San Pasqual, the Americans were in no real shape to fight. Exhausted from the journey, nearly starving, and mounted mainly on sickly mules (most of their horses having perished on the trip), Kearny nevertheless decided it was necessary to fight his way through Pico's forces in order to reach San Diego. The resulting Battle of San Pasqual was the bloodiest of the entire Mexican-American War. Pico ordered a retreat after Kearny's howitzers reached the battlefield and began firing, but in truth the Americans had suffered severe casualties at the hands of the expert Mexican lancers and were in dire straits.
Kearny's men continued on toward San Diego but were pursued by Pico's horsemen. Realizing they could not carry on in their current condition with so many wounded, the Americans set up a defensive position on a rocky hill. Out of rations, they slaughtered some of their remaining mules for food, and the hill has been called Mule Hill ever since. They stacked rocks to form a crude defense which can still be seen today. Pico's forces soon had the hill surrounded. Desperate, Kearny chose Kit Carson and two other men to sneak past the Mexican sentries at night and get help from the American forces in San Diego. The three men were successful, eluding Pico's troops and and walking all the way to San Diego barefoot after losing their boots. Within a few day, reinforcements from San Diego relieved Kearny's forces and the general was able to lead his remaining dragoons into San Diego.
The cache is hidden by a historical marker that sits just below the same hill where General Kearny was prepared to make his final stand and where Kit Carson snuck past Mexican sentries on a desperate mission to save his comrades. You cannot go up the hill itself, but you have a great view from the marker.
There are two ways to get to the cache site. One is to start in Fort Levenworth Kansas, take the Santa Fe Trail (stopping in Santa Fe to take command of the city), travel accross the deserts of New Mexico, Arizona, and California, make your way through the San Pasqual Valley while fighting deadly Mexican horsemen, to arrive starving and exhausted at the crest of Mule Hill (this the route General Kearny took). Or you can take the Via Rancho Parkway exit heading east from I15, turn right on Sunset Drive, follow it to the end and park in the trailhead parking on the right. Then cross the cul-de-sac to take the trail that winds around the driving range and the Sikes Adobe Historical Farmstead before running parallel with Via Rancho Parkway and then taking a sharp right as it leads up to the base of Mule Hill. (this is the route I took and I think you will find it much easier than Kearny's)