This thread is dedicated to tell the new players some of the things they can do in game that usually, they won't see in a guide or in the game tutorial.
Guard Mode: First line will fight, the rest of the soldiers will wait and stay in position. It also means that if the enemy unit routs, your own unit will stay in place. Good to use when outnumbered, to keep the line together and when you're on a defending position.
Transfering ancilliaries: Right click on the ancilliary and move it to the General/Familly member that you want. The two must be in a single army or in a city. However, keep in mind that some ancilliaries can't be moved (like the Evil-Mother in Law).
When the General dies, the unit moral reduces greatly (units rout faster).
Archers:
When you have selected a group of archers and right-click to attack a enemy unit, they will target different units and not only the unit you righ-clicked.
When you want your archers to attack another unit with sword instead of shooting arrows, you can hold alt and right click the enemy unit.
Merging units: You can merge units by dragging a unit card above another unit of the same type in the same stack.
If you're being besieged by a bigger army, you can attack the army, go to battle, kill some of their units and retreat all your units back to the city. After that, you can press exit battle. It will appear as a draw. You can do this once every turn.
After winning the battle and putting all the enemy units routing, you can decide to continue or finish. If you click on continue, in order to kill the routing units you can later exit the battle any time you want, just make sure that all the enemy units are routing (if not you will loose the battle) and press exit battle.
By left-clicking on an enemy army on the campaign map you can see how far that army can go on their turn.
If you want to disengage a unit that is fighting you can press w (the white flag command). That unit will retreat and disengage faster than normal. You can after right-click on the unit to stop them from leaving the battlefield. Just take care that if you are loosing and the units around have low moral it can cause a general rout.
Consider putting cheap units on the front to waste the enemy arrows on them and save your better units.
Do not surround enemy units because they will have no place to escape and will fight to death.
If you want your army to retreat from a battle and still have movement points left, then don't click on the retreat option on the battle map because your army will only move 2 points back and stay out of movement at that point. Fight the battle and tell your units to retreat. Your army will run back to your closest settlement/fort.
If you hold Shift while clicking one of the game speed buttons you can increase/decrease speed by 0.1 per click.
Always look at the settlement details window when selecting buildings to construct as the projected changes will show up in grey before the building is built. This way you can see which building will give the most income, population or trade increase without having to build it first.
For a better gameplay and in order to give the AI a better fighting chance, consider expanding slower. It will depend on the faction but for example, as Carthage, I advice conquering your first settlement after 20 turns or more. During that time you can consolidate your finances and having fun destroying the rebel armies that can be a threat to you, like the ones in Sicily and Africa. Be in mind that factions who have only one settlement are adviced to conquer as soon as possible but still do not expand to quickly.