There are 21 challenges split into 3 difficulties. If you do enough to get each of the difficulty achievements above you will get this when you complete the last challenge in the last difficulty. There are a total of 21 challenges to get this, and each will fill in with a coloured star shaped jewel to show which you have completed.
The final hard challenge was tricky and took several attempts. The enemy start with more ships and have several upgrades. Don't consider fighting them directly without the support of a town until you have upgraded your ship a lot. My preferred upgrades (when I could afford them) went Speed, Cannon x3, Speed x2, Armor last.
I start by takking Nassau directly below the start, then focus on getting resources from the small villages and boxes. Attack merchant ships to get resource steals, as the most important resource on this map is gold. Taking the central town with (the only) 2 gold deposits will help you hugely, but often gets taken by much larger ships very early on. This isn't always a bad thing, as they are destroying a fully upgraded neutral town, and you only have to destroy a basic one to steal it back.
Where other maps may have taught you to expand quickly, here it is best to stick with just one or two towns and get them fully upgraded. Once done they still aren't invincible, but the AI prefers fighting weaker towns, or just fight amongst themselves. Let them destroy each others upgraded towns so you can attack much weaker ones.
This tactic relies on the AI constantly fighting each other. Sometimes you get lucky and they keep each other at 1 or 2 towns, but sometimes one will get 4 towns very quickly. If you're unlucky there is very little you can do about it as they have much more powerful ships, and more of them, right from the start.