Detail of the Altar inside the Lower Level of Bramante's Tempietto in Rome, June 2012

The so-called Tempietto (Italian: "small temple") is a small commemorative tomb (martyrium) built by Donato Bramante, possibly as early as 1502, in the courtyard of San Pietro in Montorio. Also commissioned by Ferdinand and Isabella, the Tempietto is considered a masterpiece of High Renaissance Italian architecture.

After spending his first years in Milan, Bramante moved to Rome, where he was recognized by Cardinal Giuliano della Rovere, the soon-to-be Pope Julius II. One of Bramante's earliest commissions, the "Tempietto" is one of the most harmonious buildings of the Renaissance. It is meant to mark the traditional exact spot of St. Peter's martyrdom.

Given all the transformations of Renaissance and Baroque Rome that were to follow, it is hard now to sense the impact this building had at the beginning of the 16th century. It is almost a piece of sculpture, for it has little architectonic use. The building greatly reflected Brunelleschi's style. Perfectly proportioned, it is composed of slender Tuscan columns, a Doric entablature modeled after the ancient Theater of Marcellus, and a dome. According to an engraving in Sebastiano Serlio's Book III, Bramante planned to set it in within a colonnaded courtyard, but this plan was never executed.

Text from: en.wikipedia.org/wiki/San_Pietro_in_Montorio