Dublin Castle was first built in 1204 on the orders of King John but much of the medieval was lost after a massive fire in 1684. Visitors to the castle can tour the chapel, undercroft and State Apartments, however on this trip the State Apartments were closed as they are still used for official state visits (this case the President of France). The upside of this shorten tour was it was considerably cheaper.
The Undercroft of the castle is one of only two sections of the remaining medieval castle. It is made up of the remains of the Powder Tower that was blown up in order to avoid an even larger explosion of the powder magazine during the fire of 1684 that levelled the castle. Originally the tower comprised the outer wall of the castle running right beside the river Poddle. Beyond being the store for the gunpowder it also housed the Water Gate, where supplies to the castle were brought in. What remains of the river Poddle pool down here.
Tours of the Undercroft and the State Apartments are easy enough to find (when they are open), but if you wish to visit some of the other attractions on the site such as the Revenue museum or the Chester Beatty library it is much harder to know where to go.