The Stables control how fast your attackers and spy (espionage) agents move on the map. 1st level can be built with wood & stone while the other required rubies. When maxed it will give +120% speed. It is one of the most essential buildings that you should have in every castle and outpost (including kingdoms) and make efforts to maximize it, since faster attacks & espionage makes a huge difference for either events and wartime in any stage of the game.

Cost for Stables
Level Rubies cost Horse speed
1170 wood & 135 stone +0%
28,900 +60%
312,300 +120%