The armoury upgrades a troop’s base defense value. Every level of upgrade increases the defense value by about 1.5 percent. The 1.5 percent is only a rough guide. A fully upgraded unit at level 20 has a 35 percent increase in defense value.
The upgrade applies to troops trained from the same village. So an upgrade done in village A will not affect troops that are trained from village B. However, troops from village A may be sent to village B, and village A’s troops will still have their upgraded values even when reinforcing other villages.
Once a unit is upgraded, the upgrade will automatically be applied to all troops trained before and after the upgrade.
Once troops are upgraded, the armoury can be demolished and troops will still carry the upgraded levels.
Since armouries become redundant once upgrades are complete, it can be demolished to make room for other more important structures. However, a fully upgraded armoury generates 77 culture points. That is only a decent amount of culture points, so you may opt to keep it for culture point generation if you don’t need the space.
A common question is whether upgrades are worth it? The answer is yes, definitely. But only for defensive units.
In your defense village, upgrade all defense troops to the maximum. Scouts should be upgraded in the scout village.
The following units should be upgraded in the armoury:-
Teuton: spearman, paladin, scout
Roman: praetorian, legionnaire, Equites Legati
Gauls: phalanx, druidrider, pathfinder
The Gaul haeduan may also be upgraded if they are built in a defense village. The haeduan is a versatile unit who is good at both offense and defense, but he should be used primarily for offense.
Other units such as rams, catas, the Roman Equites Imperatoris, etc. should NOT be upgraded as they are not defense units.