3.9  DEFENCE MODE  For each land area there is a defence mode, a retreat location and ambush location. The defence mode will be one of AMBUSH, RETREAT or DEFEND. These may be changed by using the actions of the same name (see paragraphs 5.6 to 5.8). At the start of the game the defence mode for all areas is RETREAT. There are no defence modes for sea areas.

3.10  DEFEND  If the defence mode in an area is DEFEND then your armies in that area stand and fight if attacked. And area also defends if the defence mode is RETREAT but no retreat location is set (or it's set to be somewhere not owned by the same kingdom).

3.11  RETREATS  If the defence mode is RETREAT, the retreat location is adjacent and owned by the defender, and the attacking armies outnumber the defending armies and the defence level, then the defending armies retreat before the battle takes place (there will still be a battle if there are any defenders other than armies). If the retreat location is not adjacent or not owned by the defender then the defender acts as if the defence mode is DEFEND.
When a Gaelic-owned area is attacked by a non-Gaelic kingdom and a retreat is triggered then the population converts to armies and retreats as well (see 6.9 for why this makes sense).

3.12  RETREAT LOCATIONS  Retreats and retreat locations may be set by using the RETREAT action (see 5.7) but every time that an army moves from one area to another it always resets the retreat location in the area to which it moves to be the area from which it came (ie. armies always try to retreat to wherever they came from, unless you order them differently).
NOTE:  At the start of the game all areas should have retreat mode set, but no retreat location. It's a good idea to get these set as soon as possible (or replaced with ambushes, if you don't want to retreat).

3.13  AMBUSHES  If the defender has an ambush set against the area from which a MOVE, ASSAULT or OVERRUN action is made then any losses inflicted on the attacker are made before any losses inflicted on the defender are calculated (ie. a successful ambush gives the defending side first shot), the attacker suffers an extra loss in each round of battle and the area becomes sticky (see 3.7). Ambushes have no effect on PROBE actions. Ambushes do not work if the defender has no armies in the area and cannot be set in cities..

3.14  HOUSEHOLDS  If your household is located in a land area (other than a city) that is attacked then your household troops appear in that area (immediately, in time for the battle and before any retreat is decided). Once your troops have deployed from your household they become normal armies the same as any others (if you want to a build a new household you must do it the same way as before).
If your household is in a city and the hinterland of that city is captured by another kingdom then your household troops appear in the city (they don't defend the hinterland if they're placed in the city). Households never deploy at sea or in response to a city being attacked.

3.15  LOAN TROOPS  Kingdoms may loan troops to each other, from the reserve of one kingdom to the reserve of the other. A kingdom may loan troops to ONE other kingdom and may borrow troops from ONE other kingdom (but you are allowed to borrow from one kingdom and loan to a different one). Armies which are loaned out, or come back after a loan, cannot be used until the following turn.
See 6.11 to 6.14 for the BORROW, LOAN, RETURN and RECALL actions.