http://grepolis-pro.blogspot.com/
This website is pretty handy. Can't say I agree with all of his tactics, but the guy certainly knows what he is doing. Build a foundation from vet tips to develop your own play style
http://forum.en.grepolis.com/showthread.php?5666-The-Guide-to-Grepolis-(Updated)
Here is a good unit specific guide. It includes other info like researches and what have you.
My biggest bit of advice, though, is
pace yourself. I've seen too many great players or players with great potential burn themselves out by the 2 month mark, some before that, depending on the world speed.
5-min farms get you a great deal of resources, but do not become reliant on them. Once you start building sizable armies farm off shore, and it as often as possible early game (catapaults are especially great for this), as around the 1 month/2 month mark a lot of cities vanish in the server clean-up. One good swing can save you up to 1.5 hours of having to sit at the computer farming every 5 minutes.
Another thing, while this is purely up to you, prevent the game from becoming to expesive for you to play (assuming you use the premium features) by prioritizing proper city builds to avoid having to dip into your pot-o-gold because you just can't wait to smash someone with the 147 lightships you might have queued up.
Last tip, trading is important. More so between your own cities as apposed to other cities. This lets you rebuild armies quicker where you need them the most.
Sorry if this seems a but disorganized, I wrote things down as they popped into my head.
