The reason for the above sign is that non-Muslims are not allowed into Mecca (Makkah). I lived in Jeddah (Jidda), S.A. from 1974-1977. The govt. is real strict about this. This is the holiest place for Muslims. They pray towards Mecca when they pray 5 times per day.

General X is right about the crowding. Due to the layout of the different areas that must be walked to, there are areas were stampedes have killed people.

Non-Muslims do sneak in, however if they are caught, not so good for them.

Good article that gives a short explanation:
www.opinionjournal.com/taste/?id=110003943

Medina is the other holy city in S.A.

The Five Pillars of Islam on which a Muslim’s life is founded are:
-The Testimony that there is none worthy of worship except God and that Muhammad is his messenger.
-Establishing of the five daily Prayers (Salaah). These prayers are ritualistic in nature and adherence to the ritual practice is required. The location at which one prays is not strictly defined as long as one is able to establish the Qiblat.
-The Giving of Zakaah (charity), which is generally 2.5% of the yearly savings for a rich man working in trade or industry, and 10% or 20% of the produce for agriculturists. This money or produce is distributed among the poor.
-Refraining from eating, drinking and having sex from dawn to dusk in the month of Ramadhaan (Sawm).
-The Pilgrimage (Hajj) to Mecca during the month of Zul Hijjah, which is compulsory once in a lifetime for one who has the ability to do it. This ability includes the financial means and the physical strength since the hajj can be strenuous. Also, one has to obtain a permit from the Saudi government which is granted based on an annual quota based on country.

Holy Cities:
Mecca – Medina
Jerusalem
Najaf – Karbala – Kufa