14 DAY TRIAL // One of the best early things you can do for yourself and your faction in Empire: Total War is to research the Square formation for your infantry. Then, make sure you take a look at the special abilities button for your Line Infantry and make good use of the Square formation button, which makes your infantry unit move into a square, able to fire at units on all sides. It tends to be one of the most important tools needed to consistently win battles in the early part of the game.
Square formation is important because it allows fire in all directions, so you don't need to move the battalion around to face threats from different directions. Of course, there are clear disadvantages, like the fact that you are way more vulnerable to cannon fire, because all the men are bunched up in a small area and there's also the problem of less men firing on each of the four sides of the square. Still, squares are crucial in order to resist cavalry charges that threaten the flanks of an army or the rear and can also help when trying to defend enclosed locations, like forts or cities.
In the last situation, you need to put one or two units in Square formation near the center of the town, where the control flag is situated. Then, to disperse the other units, you have to cover the walls (although pathfinding is so badly implemented on walls that you're better off fighting inside the fort) and occupy all the buildings around. The enemy is going to concentrate their efforts on taking out the squared formations that are tempting targets and you'll be able to break their morale rather quickly with flanking fire and with a few quick hand to hand charges. It might not be the intended use of the Square formation but it works very well.
This is one of the major changes that have come to Empire because of the historical space it covers. In the XVIII century, complex tactics and battlefield specialization become more and more important. Changes like the Square formation, the Fire by line concept and the various munitions added to the cannon can change the way the battles are fought. Just ask the various Lithuanian armies that were destroyed via the cannonball grape shot last night.