Players can upgrade troops from any city, however, the cooldown affects all cities.
Troops can be upgraded from each city until all 7 cities are used. The system would be similiar to the multiple hero-training system. As opposed to training one hero every x-hours, we can train up to four heroes every x-hours. Troop training should follow the same format seeing as players have multiple cities, all of which have multiple (or specific) troop types.
Note -- the total population trainable per city should be costant. That is, if the Sire can train 1500 pop in one city, then the sire could potentially train 10500 pop at once. Why? Leveling up to 81 is no small feat, each city deserves the same potential.
Players will have the option to use all of their cities for training troops, which would remove the need to specify one city for training and upgrading x-troop type. In addition, the method would increase the reasons to attack all of any players' cities equally. One strategy of the game is to weaken a player's army by cutting it off from the source. At the moment it's too easy to cut off one players x-upgrade by knowing they all come from x-city. Spreading out upgrades would increase the player's ability to protect their troops, but would also increase the player's weaknesses (no longer having one city especially fortified by upgraded (best faction defense) troops.
Players *might* decrease their expenditures for troop upgrades, but being able to upgrade multiple troops across all 7 cities could more likely increase their expenditures as having multiple training bases would immediately increase the simplicity of upgrading troops and their perceived availability. Additionally, players would have to adjust their strategies to target these upgrades, but again, a change in strategy is more ideally a boost for the game's longevity as it adds another factor to overall game strategy and mechanics.