Fort Marcy ruins and Cross of the Martyrs are located two blocks north of Casa de Tres Lunas | House of Three Moons. It is well worth exploring and you will find one of the best views of downtown Santa Fe from this vantage point.
Here is some info from the National Park website:
"On August 19, 1846, three months into the Mexican-American War, U.S. Army Lieutenants William H. Emory and Jeremy F. Gilmer scaled an 80-foot bluff to survey the scene above the newly occupied Mexican city of Santa Fe. Located some 600 yards northeast of the historic plaza and Palace of the Governors, the hilltop would be the foundation for an adobe fort designed by Gilmer as the Army’s defensive hub in the new American Southwest. The site, as described by Emory, was “the only point which commands the entire town and which itself is commanded by no other.”"
"The Cross of the Martyrs memorial to 21 Franciscan priests and friars killed during the 1680 Pueblo Revolt stands steps below the ruins of Fort Marcy in Santa Fe, providing another point of visitor interest."