All four of the major cities have a number of great gatehouses - barbicans often heavily fortified guarding the entrances or main thoroughfares in and out of the city walls. Fortified towns and villages often also build gatehouses for similar reasons.
Traditionally Mercinae has a North, Northeast (river Leithe) and East Gate; Parrius has Southwest, West (river Callieton), Northwest and North Gates; Thakria has its main barbican at the South Gate, and Silverfalls has a West, South (river Silverlight) and East Gate.
High-ranked citizens, including the Barons, are able to command the opening and closing of these great gatehouses using the GATE command. Type GATES on its own for a list of your city's gates and their state (i..e. open or shut) and GATE OPEN <direction> to open a gatehouse barbican and GATE CLOSE <direction> to shut it, barring a particular entrance/exit to and from your homeland.
Gatehouse barbicans, in order for the opening and closing of the gates to operate, be sufficiently fortified - two adjacent locations within the designated gatehouse locations, typically with one containing the heavy fortifications. This defines the point of 'closure' where the GATE CLOSE/OPEN command, when issued, will take effect.
