You may choose to employ most of your resources towards an attack strategy. There are obvious benefits in this but you must also consider the disadvantages.
A strong attacker must first of all have a good economy to finance his army. A regular army is expensive to keep fit and will take up much gold and food. It is important to decide what your attack strategy will be so you can choose early on what troop types to train. Each troop type has a defensive and offensive value. This represents the ability to perform these actions relative to other troops. Some troops are more adapted towards defending and as such you may not need these. You must also take the cost per turn of each troop type into consideration. A full list of troop stats are available at the bottom of the army list, just click on the help: see troop stats link and you will get a pop up window displaying the information.
Each troop type has a fitness score. This FITNESS score shows how fit the troop is to fight. A troop type at 50% fitness is only worth half its full potential. Each troop type has a different modifier which will cause every troop to have different training times this affects the base fitness points obtained each turn. Fitness points rise every turn, so eventually your troops will be 100% fit. Officers can be employed to increase the fitness per turn. Each officer contributes to the increase of fitness per turn. A general increases fitness, but you may only have one general at any time. The training grounds plan project also increases fitness, so if you have all of these the army fitness should increase quickly as all these points are cumulative. A good disciplined army can increase fitness by 3-5+ points per turn, but this is dependent on the troop type being trained and its training modifier.
Fitness is important because every time you attack, you lose 20 fitness points. After several attacks your army will become exhausted and incompetent. It is necessary to wait between turns and recover your fitness. If you have a good officer crew this becomes an easy task.
Your army is also influenced globally by your military morale. Military morale lowers your fitness level. The highest morale score is 0 and the lowest is -60, so morale will always be a negative score. Morale rises at .1 point per turn naturally. A war win will gain you 5 morale points and a defeat will loose 2. You will also lose morale if an officer or general is killed. An army without a general may lose up to -60 morale, whilst with a general the morale will not sink below -40.
It is advisable to always have militia handy when defending, as these are counted as casualties and at a greater proportion than your specialized troops. This means that if you have militia they will be put in front of your troops to absorb most of the damage when possible.
Remember that you do not have to use all of your army in an attack. A well planned attack will involve spies to scout out the enemy forces before hand. Once you know how many troops you are up against, it is always a good strategy to attack in waves when you outnumber or outmatch the enemy. Divide your attack force into waves that can battle the enemy and this will tire him out, as you will be able to launch many attacks before your whole army is exhausted. Sending a large force will mean a great chance of success, but will also mean a greater number of unnecessary casualties caused by the over kill.
It is not always possible to attack the enemy in waves unless you outnumber him greatly. In the event that the forces are tied or of similar sizes, it is sometimes necessary to commit your whole army to one single attack. Large single occasional attacks spread out over several turns is the sure way to win yourself a slow but forceful victory over your enemy. If things don't go your way, you can always follow the attack and break the tie by employing mercenaries.
Attacking provinces without city walls is a straight forward job. If you win, the picking can be easy and the enemy can be subdued in a few turns. Provinces with walls are a different matter! To attack a province with walls, attack normally, but realize that your forces will fight at a major disadvantage. You must make sure to include plenty of militia to absorb the casualties and have a strong outnumbering force, or your attack will come to nothing. It is a good idea to storm walls with siege engines. Siege machines will destroy wall section on each attack and then help your men in the battle with tremendous battle force. Attacking a walled city with siege engines will automatically enable a special round where it calculates how many sections of wall they destroy. If enough sections of wall are destroyed, the wall becomes breached and useless. Breached walls do not count for defense. Thus a city with a breached wall is like a city without a wall. Wave attack strategy is especially important for sieges, as the more you attack, the more sections of wall you may destroy on each attack. The downside is that siege engines are expensive, are easily destroyed and cost additional wood.
A successful attacker can become prosperous and grow his province to an awesome size, but he must always be on the lookout not to become prey himself.