Garrison units are special troops whose only purpose is to protect their community. When a community musters a garrison unit, the unit remains at the community (ignoring all movement orders) until the unit is destroyed by an enemy force.
Garrison units defend their community to the death; they never retreat from combat. In battle, a force that contains garrison units will not retreat until all garrison units have been destroyed.
Garrisons will not follow anything. You cannot attach items or units to a garrison unit. If you want an item to help a garrison, leave that item in the same hexagon as the garrison (Assign Noone). The item cannot be captured unless all garrisons in the hexagon are slain.
Garrisons are built the same way as armies, but because garrison units cannot
move, they cost less than mobile army units. Be sure to defend your communities
with at least one garrison unit. If a community is left unprotected, it will
be conquered by the first unallied unit to arrive there.