The amount of troops it provides are not that great. Don't get me wrong, I never turn down free troops. However the amounts of each type of troop it provides daily is less than half of 1 single training slot in any of my buildings. Right now it takes roughly 2.5 hours for my buildings to train a single queue slot (some variance depending on which of the three training buildings). So in other words, in roughly 5 hours my barracks can produce a group of Paladins and a group of Mortars. And the total amount of troops the barracks made is going to be more than double the daily output of Dwarven Citadel.
However the Citadel set is giving so much more in addition to the troops. All these figures are for a chapter 18 city, your results may vary. Daily orc output is 86% of the daily output of a max level armory if you are making orcs on a 12 hour cycle. If you have your armory on a 24 hour cycle, the Citadel set is better. Daily seed output is 83% of what a max level Festival Merchant makes daily. Daily sentient good output is equal to roughly three 3 hour runs from a max level manufactory. Other than manufactories and evolving buildings, the best sentient good producer available is Amuni Bar. And it would take 5 Amuni Bars to come close to the daily sentient good production of the Citadel set. Finally, you are getting population, culture, and the daily Vitality Surge spell. So all in all, the Citadel set is a tremendous value for the space it occupies.