This is a pretty straightfoward progression achievement requiring gold earned in puzzles to be spent opening up new kingdoms on the map.
Kingdoms available to buy are shown as a blue shield. Those you have already bought appear as a coloured icon of some kind. Buying a kingdom sometimes reveals new kingdoms next to it that can be bought too.
The requirement for buying each kingdom increases by 250 GP a time mostly, making a total of 30,000 gold needed for this achievement. Also, you need to be level 25 to unlock all the kingdoms for purchase (thanks to Toamikel for this.) Here's the chart:Kingdoms / Gold Cost
1st = 250
2nd = 500 (Subtotal: 750 gold). At this point, you will get:
3rd = 750
4th = 1000
5th = 1250
6th = 1500
7th = 1750
8th = 2000
9th = 2250
10th = 2500 (Sub total: 13,750 gold). At this point, you will get:
11th = 2750
12th = 3000
13th = 3250
14th = 4000
15th = 4000 (Grand Total: 30,750 gold)
And the achievement is now yours!
To earn the most gold per game, you can play in "Invade" and/or "Defend" games against the AI version of other players. these are matched at or close to your own Hero's level. These games payout the most gold but gold is also lost if you lose the match. Invade and Defend are chosen from the other menu (not the "Kingdom" menu) that shows your Hero, Troops, Guild, etc.
The best way
to earn gold is to complete the tasks that pop up in the top left sidebar when looking at the Kingdom or Other menus. This sidebar can be opened with either
. The tasks are usually of the type: "Perform an action X number of times" or "Reach level Y" etc and usually pay quite well (e.g. 250-2500 gold).
Hope that helps