( I have updated my post and instead submitted saves with regular play for M19 and M20. Abusing the powers was too easy. ).
The reviews of Crusader Extreme describing it as frustratingly difficult are very much wrong.
The objective is to win.
For mission 18-20 i used the same retreating tactic: i defeated a selected opponent with assasins and then i retreated with my lord along with horses to a secluded area or just retreated along with horses.
All you then have to do is to utilize the super powers to your advantage.
Super powers make Crusader Extreme silly.
You can beat all opponents in mission 19 by initially defeating a nearby opponent, retreat and never go back to your keep again with your Lord ( i did this and laughed at it ).
I appreciated Crusader missions 33, 41 and 80 and none of the Crusader Extreme missions.
( Note: placing hovels causes slaves to set them on fire and thereby themselves and their comrades. This is to decrease the number of opponents on the screen, if your computer can not smoothly manage the multiple input, instead of wasting perhaps a planned rock volley on an outpost. )
PS! I tried M18-20 in normal play. Gamespeed starting at minimum. M18 was no big deal clearing the lions with horses. M19-20 i had enclosed keeps. In M19 you start with an enclosed keep already. I used same tactic for both M19 and M20 at start: kill one enemy + take out two outposts at start. In M20 i took out enemy in Northwest + closest Southeastern outpost and one of the two Eastern outposts ( the one with swordsmen ). After this i targeted posts with horsearchers first in both M19 and M20. In M19 i took out the Southern enemy + his closest left outpost and the second outpost to his right.
I start with Minimum Gamespeed > Granary, 3 hovels, Marketplace and Mercenary outpost > save > 15-25 assasins to enemy ( if no previous soldiers to start with ) and 5 assasins to each intended outpost to destroy > save > during the approach to targets i build defense with archers and enclose keep. You cant start any other buildings or it will take longer time for new soldiers to become available. After i sent all my soldiers and build defense, i start with quarries, etc..
I tried again after i beat M19-20 normal play and it was rather easy to again establish a good defense with this simple tatic. For example, the slaves in M19 had no chanse to approach the keep, then i had too many archers already. And i do not destroy any slaveoutposts from start, as mentioned. In M20, shields play a major part as a diversion for enemyarchers and for enemy firethrowers, as shields seem to be prioritized by the enemy. A bit of a trick! I could have only shields on a tower for enemy archers or on a wall for firethrowers, so they hurt their own. In M20 i did not destroy outpost with firethrowers to my east. They were a help, as mentioned!