The Magic Door originally stood in front of a gate leading to the nearby Garden of the Villa of the Marchese Massimiliano Palombara. Sources differ as to the date of its construction: it was either installed by the Marchese in 1655 as a monument to his own interests in alchemic esoterica, or it was built in 1680 to commemorate a supposedly successful alchemic experiment -- creation of gold from base metal -- in the salon of Queen Christina of Sweden. Christina had abdicated her throne and abandoned Protestantism, became a Catholic, came to Rome, and was a big patron of poets, artists, and, not least, alchemists -- word around town was that she had lots of base metal and needed lots of gold. The Magic Door was moved to its present location early in the 20th century.
The monument has numerous symbols and inscriptions used in alchemy, and legend has it that if you say them in just the right order (unfortunately unknown) and pronounce them perfectly you can get through the door and presumably return with goodies from another dimension. The inscriptions are hard to read from the monument itself, but they are (supposedly) faithfully transcribed on several Internet sites, some of which are listed below.
Queen Christina and the Magic Door: http://www.levity.com/alchemy/queen_christina.html
La porta Magica (in Italian):
Porta di Villa Polumbara (in Italian): http://www.esoteria.org/laportadivillapalombara.htm
Compendium of learned articles about
(part of a huge Internet site covering everything you might want to know
about Alchemy: http://www.levity.com/alchemy.)