You start off with a crew of a few vikings and try to take over the whole map. Every time you clear an island you take it under your control and can farm it for gold. On top of that, clearing an island contributes towards increasing your level and acquiring a bigger crew. Upgrading island and troop level alo contributes towards that goal.

Your aim is to get some islands, upgrade them, farm them for gold, use the gold to upgrade your tropps and gain control of more islands, until you own 'em all. The difficulty of the game isnt linear: Levels start as very simple, jump up in difficulty after 4th or 5th island then stay about the same untill the last 2 island, which are super difficult to beat. On top of all these, the boss level is easier to beat than the last 2 or 3 islands.

There are several troops to assist you in battle: Swordmen to cover your frontline, Arrows to fire from afar, Drunken Barrellmen (:D) to help you temporarily increase your troops damage through their Special Effect (explained below) and many other.
To aid your cause, each troop has a Special Effect (with its assocciated cooldown) and so does your ship. For example, Swordmen launch a massive attack against the enemy, Arrows fire x3 uninterrupted, Drunken Barrellmen raise the attack power of all your troop, Thor calls down lighting bolts to zap your enemy and so on. Having more troops of one kind increases the power of the assocciated Special Effect. So, clearly, you need at least one of each species of troops and as many as possible of those their SE you deem most usefull. Your Ship can launch a series of arrows, catapult rocks and fire explosives to further help you clear the enemy wave.

You gain gold each time you kills enemies and beat an island. That means, if you dont manage to clear the whole island, but, say, 2/3 waves, you still get some gold in compensation. You can also farm (taxate tbh) the islands you control for more gold. You can then use that gold to upgrade your islands, thus wielding more gold when taxated or increase your troops to deal more as well as sustain more damage in battle. The key is to equally upgrade your island and troops, as so to be able to claim more new territory and maximize your income for later use. You should also avoid forcing to much taxes on your island's citizens, as this will cause rebellions and reduce the island's level. To do that, after your have used the Tax button enter a battle and after you've finished your "citizens" will be ready to be taxated again.

The boss is relatively easy to beat. Despite my expectation of having 3 or 4 waves "guarding" him, you start battling with him right away. He deals huge amounts of damage and can freeze your units but between your Ship's effects, the power increase from the Barrellmen and the SE of your other units he stands little to no chance of winning against the almost fully upgraded of yours.
Bad thing is, once you beat the Boss, you dont get any gold reward to help you reach 100% upgrades. For that or if you ever find yourself in need for gold, you can put a crew of 2-3 men, taxate your population, enter battle taxate again and repeat. To be honest, it's not worth maxing everything, even for perfectionists that is.

The game has really good graphics (at least for a java game), its music is simply awesome, the upgrades system is fairly easy and the first 4-5 level epicely addictive (although the feeling wears off through repeatition). You should totally try it out, just don't stick too long to finish it. Whatever you choose to do, make sure to have fun and enjoy yourselves playing it!