Understand how the game works
The first step to defending yourself is to know what damage can be done against you, and what cannot.
- No one can destroy your City. They cannot destroy your Buildings. Everything that you build is safe.
- No one can make you unlearn Research. Once you have learned, or even started to learn a skill, it is yours to keep.
- They can steal resources. Food, Wood, Stone and Ore that you have in your inventory can be stolen if it is not protected. Gold in your inventory can be stolen and cannot be protected.
- Wilderness tiles can be stolen if they are not protected. You will lose the gathering bonus from the tile when this happens. This means you will experience a sudden loss of resource income.
- Your Might will only go down if you lose Troops. Attackers can kill your troops if they are set to defend your city against a much stronger army. You can also lose troops to desertion if you run out of food to feed them.
- Your Defensive Units can be destroyed by an attack. This will cost you any might, and it will cost you resources to build them back again later.
- There is no honor in this game. People with 25k+ armies will attack you. It's not fair. You need to be prepared to deal with someone who doesn't care about fair. The game will not stop them.
Ask them to stop
The first thing you should try when you get attacked is to ask the attacker to stop. Many people have a personal sense of honor and they will not attack smaller players who are trying to enjoy the game. High level players often attack people because they think they are idle and therefore easy to farm for resources.
Ask politely, without threatening. If you make empty threats, they will be recognized and will probably result in further attacks.
If the attacker is a member of an alliance, ask the chancellor of that alliance to ask them to stop. Alliances are usually responsible for the conduct of their members. Again, be polite.
Join an Alliance
If you are a member of an alliance yourself, you can have your chancellor ask the attacker's chancellor to stop the attacks lest they start a war between your alliances.
Your allies can also provide you with additional resources and/or troop reinforcements to help you build and protect your city.
Protect your resources
Build a Storehouse to protect your resources from being stolen. Work on your Shrinking Powder research as well. This will allow you to keep a certain amount of food, wood, stone and ore in your inventory where it cannot be stolen by an attacker.
If you have more resources than you can protect in your storehouse, consider moving the extras to your other city if you have one. Or, you can even hide some of them in a wilderness space if you think that wilderness is less likely to be attacked than the city.
Protect your gold
Good Gold management is key to saving gold. Use the Increase Gold strategy. This keeps your knights employed without creating a stockpile of gold. - when you need gold, go to your castle and use the increase gold button. Immediately spend that gold: buy your latest research, or put it on the market for some resource you need. Do not leave it lying around to be stolen.
If you know an attack is coming from your Watch Tower and you have gold in your inventory, go to your Market and buy any resource that is currently available for sale. The idea is to spend your gold on a resource that will then take 30 minutes to reach your city, instead of losing it for nothing to an attacker. While recent game updates mean that all market transactions are immediately cancelled as soon as your city comes under attack; once you have completed the sale and the money has been given to the seller, the transaction cannot be cancelled. Thus, be sure to match an existing offer price.
Also reinforcing a wilderness can be quite usefull when your city is under attack. You can reinforce your wilderness with supply troops or wagons, carrying the gold from your city, so this will not be stolen during the raid. For more information read "protect your troops" below.
Protect your troops
Put your troops in Sanctuary. Never defend your castle with your troops in your city when your not there to change it under attack. Additionally the choice to defend or not defend is a not even available without a level 8 watchtower, as this is the only level you'll get detailed information about an incoming army. Don't waste the troops trying to defend unknown army numbers, you'll just lose might when they die to overwhelming odds. Defensive units won't cost you might, but they are still mostly a waste of resources better spent on city upgrades.
None the less one option to defend does exist to deal with the persistant farmer, with all the troops reinforcing the wilderness (carrying the gold and what your storehouse couldn't hold), you're free to set your castle to defend by clicking the castle and selecting "order troops to defend city" with no risk of losing your troops. (choose a semi-distant level one wilderness; players who click it won't be able to beat what you have, and choose anything but a plain, even a higher might player would click a lev 1 plain to build a city!) A distant wilderness will make it harder to find by players looking to take out your wildnerness holdings.
The reason to defend is simple; with all the goods on the backs of your troops and city set to defend (I like to call it meatgrinder) the person farming you will lose might every time he attacks you and will walk away with nothing. Sure, you'll lose castle defenses, but you can rebuild them and it's a small price to pay to get the local bully boy to never farm you again. To further stymie the effort to farm you, do this when you log off for the night, (you're troops can stay in the wilderness indefinately with no penalty) set that tax rate to bare minimum (just enough to stay positive) because you can always use the "increase gold" option when you log in again. I threw a player a 10k might loss with this technique, (with only a third of available defenses built) and for his trouble he walked out with 78 gold. Needless to say, he never tried it again.
Wait for them to go away
Follow this advice, and most people farming for resources will move along when they can't get anything from you. There's no point in repeating an attack with no profit.
Once you reach 300k might or so, you should be strong enough to defend against attacks from other players. Build up an army of 30k troops of your own, and start working on your Defensive Units to support them. Then you can start defending your city if you like.
Wilderness Ping-Pong
A weak player can still take revenge on a stronger attacker. Remember that players with lots of might have a constant need for food to feed their large armies. If you steal the Grasslands and Lakes that they own, that will cause them to lose food income. Very large armies can lose so much food that they start to starve, and the troops will desert from the attacking player.
Most players do not defend all of their wilderness tiles. They simply cannot afford to keep one army stationed at each wilderness, because then they would have no slots open in their Rally Point to send attacking armies out. Players who do defend their wilderness tiles usually only leave a token force of a few hundred small troops, just enough to deter a small attacker from sniping the wilderness away.
A Sniper Attack is the fastest way to steal a wilderness, and it can be done by a low-level player. First, locate a wilderness that belongs to your enemy. Scout the wilderness to make sure it has no troops (or skip this step if you want to make sure the attack is a surprise). Then send a small army of 1 Scouts or a few Cavalry to attack the wilderness. Use scouts or cavalry because those units move faster than most troops. As soon as you conquer the wilderness, reload the game if necessary and recall the troops back to your city so you don't lose them in a counter-attack.
If your scouts turn up a small defensive force, you may be strong enough to defeat them with a larger strike force. Plan accordingly.
This turns into a game of ping-pong when the enemy counter attacks and steals the undefended wilderness right back from you. Obviously this strategy is much more effective if you believe the enemy player is not online and therefore cannot take the wilderness back from you before losing a significant amount of income.
The ultimate way to win this game of ping-pong is to lose: Snipe the wilderness, and immediately abandon it. It will revert to an uncontrolled tile, and the barbarian army will build back up over the course of the next hour. If you can snipe and abandon a wilderness this way more than an hour before the enemy can retaliate, they will be faced with fighting the barbarians to take it back, instead of facing your forces.
See the Wikipedia pages on Guerrilla warfare and Sieges for historical examples of this type of tactics.