In real time strategy games, it is impossible to defend (or "turtle") all places at once effectively. If you are defensively minded you must be capable of moving your units to a common point to defend against an incoming army at any time.
Because Evony is in real time, however, you may not have the privilege of even knowing when your enemies are attacking, as you may be asleep while a nearby enemy only has a 2 hour travel time to your city. You can build all the towers you want, but someone can out-produce archers or warriors to overcome your defenses.
While outward expansion may give you an advantage of easier reach over resource valleys and more potential targets, you are left less able to reinforce cities in a sudden panic even with maxed out relief stations. It may seem logical to find distant areas to settle to expand your empire, but it can become a nightmare against a strong opponent.
Consider the Hub region and Raiding city approach instead.
Begin by building up production towns around a small area of land close to your Hub city. Those towns will work to produce resources and gold to support your research and army. Enough towers are needed so that nearby neighbors are discouraged from attacking you in favor of easier targets. As a general rule overall, keep your resources tightly spent so there isn't much to plunder.
Take note of the alliances nearby. Go to the map view, select the coordinates box, and use the arrow keys to move four clicks in a single direction away from your hub. This four-click radius represents about a 2-hour marching distance around your town. Look for prominent alliances within this range. This can be important, as your neighbors can easily identify your cities. If there is a dominant alliance in your area it may be best to join them as they are a credible collective threat against your Hub.
Never, ever use hub towns to initiate scouting parties or to launch attacks from. The key is to not annoy any neighbors. Contact them and let them know that your policy is not to be aggressive toward them, but that you will defend yourself and retaliate against any attacks. Your hub must continue to be a secret. For this reason, do not attack anyone within about eight hours from your hub, and never launch your attacking forces from the hub towns. For that purpose, you will be using the Raiding city.
Once your military production and economy is on track, make sure to keep 1 or 2 town slots free for Raiding cities, depending on your strategy.
Identify potential target areas for raiding by their city level, honor, and nearby reinforcements. Choose opponents at least 8 marching hours from your hub. Select a barbarian valley or the valley of a non-alliance player and capture it, creating a city. Immediately reinforce keeping in mind the initial build cost for rally points, feasting halls, army food/gold upkeep, and a few levels of your beacon tower.
Your entire imperial army can focus on this town and use it as a raiding center.
From your raiding town, select enemies who are not part of an alliance within close view of your hub. The last thing you want is an enemy complaining to his alliance that he's being raided and someone else piping up about the coordinates of a big bunch of your cities. Your raiding town is your power center, but it is fed by the generator of your hub towns.
Archer towers are not necessarily important for your raid town. Split your attacking army so it is enough to overcome the enemies you are after but certainly not overkill. Your hope is that enemies will counter attack assuming that the number of troops that hit them are your entire army. You will gain massive honor when there is a larger army waiting in your raiding town.
Keep a strong scout wall. Against early players on new servers 1000 is more than enough. Tough players require 3000. Late server play keep as many scouts as you can. If your scouts foil enemy attempts at intel I have noticed they will assume your army size is equal to the earlier raiding force and attack blind. This will gain you massive honor in defending.
Now for overall strategy.
Keeping 1 town open compared to your rank will allow you to create a raiding town. You can plunder nearby enemies until they are dry and then melt it down to gold on the market.... shipping it back to your main cities (much lighter to transport).
When the area is obliterated you can move your army back home and abandon the town. You will not lose honor for this. It's best in this mindset to avoid over spending. Your army is your defensive shield and you really only need enough infrastructure to support your hero/es, rally points for the size of the army, tech to give bonuses, and relief stations to ease transports/reinforcements.
With a two town mindset, you may establish a new town in enemy territory and then capture an enemy town with your second extra slot. You may use the new town are your new raiding base, abandoning the previous one. This can create a hopscotch sort of way of playing war... Alternatively you can take over enemy towns, ship out all the goods you can, and then abandon them. You can clear wide swaths of enemy territory this way without the negative of having to defend the new city.
Ultimately, keeping a collection of productive cities with good defenses that never scout or attack secret from opponents can be a massive benefit. Few opponents are obsessive enough to comb the entire landscape of Evony to find your main hive... and those that do will be 8 hours out without siege weaponry. You will have enough time in those circumstances to send your primary army back from the raiding city to the defense point... or even to move all your resources from the attacked city to another.
Make peace with your primary neighbors, and attack by proxy with raiding towns. You'll find greater success in Evony as a result.