It is mentioned in the editor's preface to Ibn Abdul Wahhab's Kitab Al-Tawhid that the people had fallen into such polytheistic practices such as the veneration of graves, stones, caves and evil spirits; all those things that the prophet Muhammad, salla Allah alaihi wassallam, had been sent by Allah, subhanahu wa Ta'ala, to eradicate.
Amir Muhammad ibn Saud greeted Muhammad ibn Abdul Wahhab and said "This oasis is yours and do not fear your enemies. By the name of Allah if all Najd was summoned to throw you out we would never agree to expel you. Muhammad ibn Abdul Wahhab replied, "You are the settlement's chief and wise man. I want you to grant me an oath that you will perform Jihad against the unbelievers, in return you will be the imam, leader of the Muslim community and I will be leader in religious matters. (1) Amir Muhammad Ibn Sa'ud presented himself as a student to Ibn Abdul Wahhab along with some members of his family. Dir'iyyah became popular as a place of learning and as such was transformed into a centre of Islamic education and Da'wa and people began migrating in great numbers.
The Dir'iyyah area is now undergoing extensive restoration as can be seen from the photographs; some parts are still virtually in ruins and are deemed unsafe to venture into whilst other parts have already been rennovated. Original building materials would have included dried mud bricks, limestone, wood (athil), palm leaves and branches. (2) The restoration has been very faithful to the original style of building. In addition to building restoration there is plenty of green space on which to picnic and trees offering shade. A very welcome alternative to the usual chaos of Riyadh city centre. I took the following photographs back in October 2007.
Further online and offline reading:
Dir'iyyah and the First Saudi State by William Facey