Here is a list of some basic strategy involving Ultimatum's battle system. If you want to suggest something to add to the list, feel free to contact me.

Most maps have at least one village that is right beside a castle. Take advantage of the fact that your leader can move at the start of the game, and capture that village after you finish recruiting units.

Keeping your leader at your castle keep is key to win in modes such as War and Capture the Flag. Unless otherwise stated in game modes, keeping your leader in your castle will help you stay alive longer. You also only can recruit when your leader is at your keep, so you have no excuse for a shortage of units.

It's a good idea to have a line of defense because skirmishing units can easily go through attacking units. This is even more important when you don't have many captured villages, or the map doesn't have many villages. Keeping at least 2 or 3 (dependant on the game mode and your situation) units in between your point of attack and your castle is a good idea in case any units manage to go through your attacking forces.

You should be checking the weaknesses and resistances of units, so you can use it against them. Skeletons are weak against fire attacks, so using a mage's "fireball" attack against them is effective. Since the skeleton doesn't have any ranged attacks, it can't defend against the fireball attack. But beware, the same goes the other way. A mage is weak against blade attacks, and has a very weak melee attack for the Skeleton to take advantage of.

At the start of the game, you should be recruiting your fast-moving units to capture all the villages you can. The key to victory could have been those extra 2 gold for that mage you wanted to buy.

Most units have higher defense in some terrain than others. A forest generally gives out higher defense than a flat grassland, so attacking from a forest will give you the extra advantage. You also should take advantage of the defense of a village, since the village recovers health and supplies every turn for your unit.

If you have a healing unit availble, use them frequently, and prevent them from getting into combat, as they are weak in melee. Healing units will win you battles without needing to attack themselves. On the other hand, if you see a healer on another team, you should bring it to your top priority.