Historically speaking, I still think Persia would be the best choice. Not saying it would be my favourite choice, however it would be historically appropriate.
It is only possible through a bug that was supposedly "fixed" in this last update. I imagine some people either slipped through the cracks, or they just fixed the bug itself, and didn't remove the extra buildings/walls from those that already benefited.
I just build an entire army of Fusiliers (The main melee troop, yours might be called Pikemen or something different depending on Race and Age). This only works because I keep my medals low on purpose so I can raid people at a lower level than me. The army is super cheap and builds quickly...
When attached to a wall, it should auto-snap. If it's snapping to the wrong wall, remove the one it's snapping to, place the gate, and then re-add the removed wall.
I agree with what most everyone is saying - that the rewards for league bonuses are minuscule to say the least. However, I don't really see an issue with the old man's quests for 2-5 crowns. I don't find them overly hard to accomplish, and generally they can score you an extra 20 crowns easily...