Ok, if you are not into the game, but are saying "OK, this isn't a fad and these people are having fun. I WANT IN! " No fears, simply click here, hit the number it says, click on "Join the War!" on the next page, and register and you are in. Look at other "KoC:" JEs from others playing the game to get the strategies and stuff down...
Why D is better than O: Its a matter of math. In 1 day, you get 48 attack turns. That's 3 full attacks. How much cash can you get in 3 full attacks? $50k? $150k if you are lucky?
With as many troops as I have (as humans) I get over $16.5k per turn, and you have 48 turns per day, that's $792k per day. And I get at least 8 new troops everyday, which means my $ per turn increases.
So I have towers and almost 100k defense. I'll keep at it until I can save the whole 650k for battlements. I may never go offense until I can get everything up to 'the hand of god.'
Oh, and 'saving helmets' is not the best way to go when saving up for big things. Cause if you get attacked 20 times overnight, you could lose every helmet to degredation. Better to save something like platemail (your 640 items) so that you can stand an onslaught without losing your investments.