The Black Gate separates the Old City from the New.  More significantly, it separates those who have wealth and power from those who do not.  Consequently it is manned by an elite patrol, wtha standard patrol always stationed in the gate house.

  The Black gate is closed two hours after sunset and remains closed until an hour before dawn.  When the gate is opened a roster is diligently maintained and the guards keep a sharp look out for fugitives.

 A roster is kept of those passing through and those arriving in the city are asked their business.  Those carrying weapons other than: dagger, dart, sling, staff, staff-sling, club, and knife are required to pay for a license from the Captain General of the Watch, there are no licenses for two-handed weapons.  A Freesword Tax of 3 gp is also payable for anyone seeking mercenary work.

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Each gate comprises a pair of huge wooden portals, each at least 12 feet high, eight feet wide and one foot thick.  The Black, Highway, Garden and Cargo gates are 16 feet high and 24 feet wide.
Each gate is backed by a massive portcullis of iron bars 2 inches thick.
Each gate is enclosed in a small gatehouse consisting of a pair of towers flanking the gate and an enclose area over the gateway.  the tower tops and connecting blockhouse feature arrow slots and murder holes for boiling oil.

Gates of Greyhawk