First, move your flag into the bottom (or top if you prefer) cornet. Corner it in with two walls. Don't forget the e and r buttons (I think those were the two anyway) can rotate your walls and later your marshes. Place the walls so they look like:
_
x|
With x being the flag. Then, imagine where the two corners of the walls meet there's a line that looks like /. Place your cannon(s) on that line. You them to be to the right of your rightmost vertical wall, and above your highest horizontal wall so that your cannons don't end up shooting your own walls when trying to kill raiders.
With your walls like that and a cannon you'll be set for a couple rounds, just sit and keep going until it tells you there's a new unit: Masons. These

holes want to destroy your walls and there's nothing you can do but pray your cannon just happens to shoot enough of them down that they don't overwhelm your walls. When you're told that masons are coming, build two more vertical and horizontal walls. That should be enough. Next round you'll want 4 each, and the round after you'll want 5. After you have 5 good walls, try to build another cannon. I forget when I had enough, but it was around this time anyway.
You'll also get access to marshes and warriors around this time two. Ignore the marshes for now, don't even bother with warriors at all. (Reading the description of the warriors tells you to keep them away from your cannons. To me, that sounds like your cannons will kill your warriors if they happen to get in the line of fire, so I didn't even bother, if it turns out they are ranged and not the melee units I assumed they were, they might be decent, but then I'm also gonna assume that the assassins you face later on will just kill your warriors dead then anyway)
By level 15 or so, another unit comes along that you will learn to hate with a passion. They are blue, and I forget what their names were, but they are like the masons, only they target your cannons instead. Yes, they will target your cannons while your cannons randomly shoot at other things. Sometimes your cannons will shoot them down. Hopefully you will be lucky enough to never lose an entire cannon in one round. If any of your cannons get damaged but not outright killed, just sell it any buy another one.
By this time you should be working on keeping your wall maintained, and working your way up to the 10 cannon limit. You'll gain access to whirlpools, but don't worry about them quite yet. You'll be close to 10 cannons when you get them.
I think it's level 20 when the warships start coming. They are harder to kill than the normal ships, and they have cannons of their own to attack your walls and your cannons. I had something like 7 or 8 cannons by this time, so it wasn't too bad. You should be clearing the levels pretty easy if you made it this far. Once you get 10 cannons, now you can go wild. First, use your whirlpools every turn. You get to place two and they go away at the end, so make sure to keep placing them. As for placement, place the edge so it's touching the right edge of the screen, and place the top one about 1 1/2 ship heights down from the top of the screen. Place the second one just over (but as close to as you can) 1 ship height below that one. Put them there every turn. Now, place any remaining cash you have into marshes. Put them along the shore (though keeping in mind there's no need to put them along the top half because your whirlpools are taking care of that area) and up to your cannons the way your walls are built around your flag. But there's no such thing as too many marshes. They're the cheapest thing to build, and hopefully you'll have enough that when pestilence comes (level 25), the random thing taken will be your 1,000 gold marshes, and not your 15,000 gold cannons, or either of the whirlpools that you desperately need now.
Once you hit level 25 though, you're in decay. You'll last a couple more levels, but eventually the war ships start building up (by now, you'll want to keep the cannons clearly separate from your walls so any warships aiming at your walls don't hit your cannons instead) and a whirlpool or two will be taken by pestilence and you'll lose because you can only have 10 cannons at once. Lame.
That's as far as I got, maybe something happens after that, probably not though.
If you do decide to play though, good luck.