I only play in one world of Grepolis, yet I managed to get in the top-50 of this world. This is not a bad result and many people just wonder how I managed to do this. So, here is some information about the strategy I’ve used.
First of all, I have a premium account, which allows me to pay for the five advisors. These advisors are really important when you want to grow fast, since the administrator will allow you to start building more buildings. Time is important and it’s very practical when you can start building 7 buildings before you go to sleep, to notice they’ve all been build the next morning. Without the advisor, you’d only have two new buildings by now. The merchant just increases your resource production, allowing you to build sooner since you get the resources sooner. The high priestess is practical because she grants more favors, allowing you to throw spells more often. And some of those spells are very practical. The commander just makes your armies stronger in battle. And the captain makes your ships faster, allowing faster trade with other cities.
Next, make sure you start increasing your production as soon as possible, up to the maximum levels. If your timber camp, your quarry and your silver mine are at the maximum level of 40 then you will earn the maximum amount of resources, allowing you to build a lot faster. Time is the most important factor in this game. The sooner you can build something, the better. Additionally, bring your marketplace up to level 10 or higher so you can send your resources to your other cities or trade them with other players or local farms. And when your production increases, make sure your warehouse can contain it all.
Next, make sure that you build a new building every day in every city you have. This is a minimum requirement but also makes you grow faster. This building should be something like a city wall, temple or academy. (Or one of the special buildings.) Then, if you still have resources left, apply a method of opportunistic building, where you just build anything that can be afforded by the town. In general, the resource producing buildings tend to be cheapest thus you’d be building those most of the time. Once you cannot afford a new building, send the remaining resources to another city that could make good use of those resources.
Whenever you have one or more towns on an island with farms, make sure one of those gets a merchant shop. This town also needs it’s warehouse and marketplace at the maximum levels and will basically become your “bank” for this specific island. Other cities can send resources to this merchant town, which in turn will be sent to other towns, or traded with other players or the local farms. It also has a trade advantage with the Phoenician trader.
Next, attach and defense. In general, players who attack a lot of other players will make too many enemies, putting their own positions at risk. Attacking other players also costs a lot of resources and often isn’t very profitable. When the world is new and you need the resources then attacking inactive players will provide some additional resources. Sometimes, an inactive player has a nice town to conquer so go ahead and conquer it. But if you try to attack active players to conquer their towns, be aware that they will do everything possible to prevent this. And if they’re part of an alliance, that whole alliance might turn against you. So be careful in picking your fights.
And defense? There are two possible enemies. Those that share the same island as you, and those from other islands. If you have enemies on the same island then make sure your land units are strong enough to protect your cities. (And remember to use the militia when under attack!) A standard defense should have 100 swordsmen, 100 archers and 200 hoplites. Add some additional chariots or mythical units to this just to make them stronger. To protect against enemies overseas, add biremes to your harbor. And you will need a lot of them too. Just 50 biremes is not enough.
A better defense can be achieved by having your towns close together in groups. In this case, when one town is attacked, the other towns can send defensive units to strengthen the defense. Also try to eliminate any enemies on the islands you occupy, either by conquering them or by joining their alliance. Or at least befriend them. If there are no enemies on the same island then the attacker will need to get through your biremes first. They also need to add transport ships to their attack, decreasing the number of land units that can be part of this specific attack. Often, they would need to send two attacks. One to clean your harbor and the next to attack your land units. When this happens, you can let your biremes dodge the first attack, returning just in time to stop the second attack. Thus, these kinds of attacks tend to be very unsuccesful, unless you take the defender by surprise.
Many players will demolish some of the buildings in their towns just to get a few more residents. This will also decrease your point value and I consider this a waste of resources. Yes, maybe this allows you to attack with 60 horsemen instead of 50, but is it worth the waste of resources and points? I don’t think so. It’s more practical to just build a second town and use that one to produce additional units. If you use one town to create defensive units which you use to support a second town, the second town can just focus on building offensive units without the need of defensive units.
But most important: be online every day! Preferably two or three times per day. First of all you will get the daily bonus this way but it also allows you to check all your towns, making sure they’re all safe and all resources are well spent. (Or stored in your bank, without flooding the bank.)