Trajan's Column Historical Timeline

A victory monument that became a tomb, lost its emperor, and was given a saint instead.

Chronology

1. 101–106 CE — Trajan conducts the victorious military campaigns against the Dacians that the column exists to celebrate. Structural impact: none yet — but the campaigns are the column's entire subject matter. What you see today: 23 turns of relief narrating them.
2. 12 May 113 — the column is completed and dedicated, probably constructed under the supervision of the architect Apollodorus of Damascus at the order of the Roman Senate. A gilded bronze statue of Trajan is placed on the summit. Structural impact: the monument as built — shaft, frieze, internal staircase, statue. What you see today: everything except the statue.
3. 8 August 117 — Trajan dies. A golden urn with the emperor's ashes is placed in the base; Wikipedia adds that the Senate voted for the burial and that his wife Plotina's ashes were set there too, in golden urns. Structural impact: a triumphal column becomes a mausoleum. What you see today: the square base, empty.
4. Middle Ages — the statue of Trajan on the summit disappears. Structural impact: the column loses the figure it was built to carry. What you see today: the reason there is a saint up there instead.
5. 4 December 1587 — Pope Sixtus V crowns the top with a bronze figure of Saint Peter, which remains. Structural impact: a Roman imperial monument given a Christian head. What you see today: St Peter, 30 metres up, on Trajan's column.
6. today — the monument stands on Via dei Fori Imperiali, owned by the Sovrintendenza Capitolina, with no published hours, no admission and no published interior access. Structural impact: none. What you see today: the best-preserved continuous narrative of a Roman war that exists, of which you can read about an eighth.

• Official website
• Wikipedia