Granary is in many respects a lot like Well, though I see Well as the more representative of the particular game mode than Granary, maybe because it’s more structured. Anyway, I wrote the Well guide first, so I’ll just copy the bit on objectives over here. Both Granary and Well have what I call the “tug of war” game mode. Basically the maps contains 5 linear control points, and each team starts controlling 2 with the middle one beginning neutral. Both teams fight of the middle point, and once it is captured, only 2 control points are in play at any one time, both teams’ furthest forward one. The ultimate goal is to capture all 5 control points in order.
A couple of things worth noting that mix up the achievement of this objective however is firstly that each control point has a different capture time – that is the middle point takes the longest to catpure, whereas a single player can capture the final points in around 5–10 seconds. Also holding more than your two initial points (i.e. controlling at least the middle, maybe some enemy points) grants your team a decreased respawn timer and allows you to spawn further forward, reducing transit times to the action. See my article on map control or a look at the importance of this.
Unlike Well, Granary is very flat – most of the fighting takes place on the ground floor – and there is no setup time, so the scout rush is very prevalent.