One of the oldest parts of Riyadh - the fort itself is free to enter (just sign your name in the guest book).  There is a comprehensive history of Riyadh dating back to the 1930's.  All exhibits are explained in English as well as Arabic.  As Masmak fort is adjacent to Dirah Square, it probably isn't a good idea to visit on a Friday (Dirah Square is infamously known as "Chop Square") as this is the day when any punishments take place publicly.  The car park adjacent to the fort has a small charge - watch out for the numerous men who will rush over and offer to wash your car for you!  You are also just across the road from Batha Souk which is well worth a visit - selling everything from complete tat to gold, pashminas, rugs, coats etc.