Movement

ai_turn(civ) in CONTROL.C moves a civilization’s units. It is used for every civilization. For the human it reads commands from the keyboard, mouse or a unit’s standing orders. For the computer it takes them from ai_order(). Directions 1–8 index the first ring of spiral_x and spiral_y, starting north and turning clockwise.

Movement points

Movement is counted in thirds. At the start of the phase every unit gets 3 * utypes[type].moves thirds, plus three for ships while the owner has the Lighthouse or Magellan’s Expedition. A unit that was fortifying becomes fortified and does not move that turn. Sentried and fortified units are skipped.

A land unit’s step costs:

Air and sea units always pay 3 per step.

The railroad rule is narrow. Only the human’s manual moves are free, and only the starting tile needs a railroad (both need a road). A unit on a go-to order, and any computer unit, pays one third. A river is ordinary terrain with a move cost of 1; it gives no road-like bonus.

Moving with too few points

When a land unit with anything other than exactly three thirds left enters terrain that costs more than it has, rand_scaled(3 * move_cost) > moves decides whether it fails. On failure it loses its remaining points and stays put. A unit with exactly one full move (three thirds) always succeeds; so does a unit whose remaining points cover the cost. After a successful step the cost is subtracted and clamped at 0.

Zones of control

A land unit (other than a Diplomat or Caravan) cannot move from a tile next to a foreign unit to another tile next to a foreign unit, unless the destination holds units. if_open() treats a city tile as never blocked, so moving into or out of a city is always allowed. Units at sea are exempt. The human hears a sound; with debug bit 0 set (as at Chieftain) the *ZOC help text also appears.

unit_blocked() performs the test. When the human is the one tested, it also reveals the foreign units and cities that are doing the blocking.

Entering a tile

Moving onto a tile occupied by another civilization’s units is an attack; see Combat. Other rules, in the order the code applies them:

• Diplomats entering a foreign city open the Diplomat menu, and moving onto foreign units tries to bribe them (buy_u()). Neither Diplomats nor Caravans may enter a foreign city from the sea.
• Caravans entering a foreign city establish a trade route. Entering one of their own cities, they offer “Keep moving”, “Establish trade route” and, when the city is building a wonder, “Help build WONDER”, which adds the Caravan’s cost to the city’s shields. At a map_dist() below nine from home the trade route is dimmed, and a city on the home continent, at a distance below nine and not building a wonder, is simply entered.
• Boarding. A land unit can enter an ocean tile only if its own ships there have free capacity (stack_cap()). It is sentried, and boarding uses up its movement.
• Ships cannot enter land other than a city.
• Foreign cities can be entered by land units only, which captures them (capunit()). A Nuclear unit entering one detonates it.
• Leaving a ship onto land ends the unit’s movement.
• AI stacks. A computer unit outside a city will not join a stack of four or more of its own units (two or more when it has no movement left).
• Turning back. A computer unit that would step straight back the way it came, in a stack of at most two, stops and drops its destination.

Territory

When a city is founded, city_create() writes the owner’s number into the site layer of page 2 at (x + 80, y) for its 21 working tiles. It also lowers the site values of the next ring (to 8). A land unit stepping onto an improved tile (with a road, irrigation or mine) owned by another of the civilizations 1–7 can cause a diplomatic incident:

• The human is asked to confirm breaking the treaty (attack_ok()), and confirming cancels peace.
• A computer unit at peace with the human will not enter the human’s territory.

The test applies only between the human and others. Units on improved tiles of foreign territory become visible to its owner, and any other civilization’s step clears the ownership mark.

Carrying units

A ship with capacity, or a Carrier, moves its cargo with it. Land units travel if they are sentried, if the ship starts at sea, or (for computer players) if they have no fortify orders. A Carrier moves aircraft instead. At most capacity units go with a ship, and Barbarian ships take everything. Carried units are sentried and aircraft are refuelled.

Huts

has_hut() derives huts from the map seed: at most one per 4 × 4 block, at a position chosen by map_rng_state, about half the blocks, not on ocean, cities or the polar rows. Visiting a hut sets bit 0 (the Barbarians’ bit) in map_visible_to for the tile, which removes it.

enter_hut() in UNITS.C ignores Barbarians and aircraft and draws rand_scaled(4):

Mercenaries are a Legion or Cavalry unit for the visitor. Barbarian units from a hut are visible to the visitor.