Please aide me in my quest to understand the battles of MTW..

I have just failed a campaign as Portugal in XL, but unexpectedly, I shall explain shortly this time;

I did not want war at all with the Castilians, and after losing a campaign as the Castilians themselves, (Which was going very well), to a sudden attack by the Almoravids, I wished to keep a peaceful border on all fronts.

So I planned to make Portucale an impenetrable city-state until I had enough currency to send an army someone, and until then make my largest surplus through crusades.

I built the greatest amount of trading buildings I could, upgraded the farmland to 80%, and meanwhile I created an army thus:

3 Urban Militia

4 Feudal Sergeants

2 Spearmen

3 Archers

3 Royal Knights (Royal family & King Henrique II)

It was the most I could afford up until my demise, each unit had 1 unit of valour.
When the Genoese were excommunicated, as they always are, I sent a crusade to Genoa.
Unfortunately by the time the Crusade reached the Provence, the Genoese Consul had eradicated the Papal authority.

I had all buildings that could financially assist me built, and decided I would continue to crusade towards Serbia until my Castle was built in 12 turns.
Meanwhile I kept my garrison as strong as mentioned, it consisted of 1021 men, and attempted to ally with everyone.
But NO ONE, not even the Pope previously, would ally with me.
Only Serbia when they heard of 4360 devout Catholics marching to Croatian Illyria.

Suddenly, whilst my crusade was approaching Navarre, (I love that province!), the Almoravids attacked with 960 men with 400 reinforcements, led by the Khalifh himself.
It consisted of roughly;

5 (Muwahdi?) Infantry

4 Almohad Urban Militia

5 Desert Archers

1 Peasant company

4 Ghulam Bodyguards (Khalifh & Co)

1 Ballista crew

Ha ha, I love my memory!

Anyway, I positioned myself on a small, but very steep eminence with a forest directly adjacent to it, halfway down the slope.

I placed my Sergeants thus :

- - -
-

And position my spearmen on the right, with my King behind them to fill the formation perfectly.
To fill the left, I placed the archers.

Then halfway down the hill I placed the Urban Militia with the Princes in front.

The enemy engaged, the spearmen both engaged two Ghulam Bodyguards downhill with the King attacking the right flank as they did.

The Sergeants flew down the hill as the enemy traversed the steepest part and engaged the Muwahdi evenly, the archers, being out of ammunition, engaged a third party of two Muwahdi as the rushed to join the battle from my right.

Below this, the ambush from behind was stopped by the sheer amount of Urban Militia that, although ambushed and attacked on their flank as they attempted to engage the Sergeants, tore apart all my forces in the forest.

It seems no matter what I do, I always see my forces, 'Losing Badly', even though they are given every advantage always.

The battle was fought for about 8 minutes, with my troops losing constantly, and suffering over 50% casualties.
Suddenly, the enemy, still fighting uphill, lost, and the centred battle gave way to the Sergeants, who chased the enemy down the hill.
Then the Almohad militia routed, and all the Muslim militia began to run.

The Ghulam bodyguards however, 40 of them, had killed 200 spearmen and 19 Royal Knights whilst surrounded and fighting uphill against a wall of spears, for them victory was never in doubt, they lost 10 men.

My King ran away with 50 men.

In the following siege of the castle, all but 78 of the Almoravid army was killed by the Ballista towers attempted to break through.. Ridiculous.

The province rebelled and there was a small battle of 128 v 78 to fight.

By this time I was doubtful I could win even that, so I came here...

I love the battles in this game, I do really, they feel far more satisfying, and I love the idea of losing for once even though I outnumber the enemy, or have a good fortification, but I can't enjoy it if I am left in disbelief..

Why?