Unfortunately, due to technical reasons beyond my control, I cannot post the cost calculations for building troops in two castles on the forum today. I will do this on Friday. In general, building up to level 7 creatures, without any upgrades, in the Castle is 15,000 more expensive (37,000 - 22,000).
Of course, this includes essential buildings for construction, such as the level 1 mage guild (for both cities), the forge, and the stable (for the Castle).