The Dome of the Rock is a sacred monument in the centre of Jerusalem, and is used in Age of Empires II. It is seen in the Barbarossa and Saladin campaigns. Most times it is seen, it is the focal point of an objective.
The Dome of the Rock is seen throughout the Saladin campaign, and in the final mission of the Barbarossa campaign. Usually, destroying the Dome of the Rock is a cause for defeat, regardless if the team controlling it is allied or enemy to another team. The team controlling it is always called Jerusalem, always represented by the colour blue.
The Dome of the Rock cannot be built by villagers, much like the Cathedral and Trade Workshop, and like both can be found in Create Scenario. Curiously, the Dome of the Rock is found in the "Other" category of units, rather than "Buildings", which is where the Cathedral and most other non-buildable buildings can be found.
Following from that, the Dome of the Rock can be available by converting. It doesn't hurt to try. (Unless you have max buildings, which then the Dome of the Rock will collapse and you will lose).
See also Edit
Other non-buildable buildings: