This is not not intended, Mongols should be able to keep their settlements, at least for some time. I've even drafted a code to make them easier to achieve so.
Code:
; STONE WALL - HAPPINES & LAW
law_bonus bonus 1 requires event_counter is_the_ai 3 and hidden_resource capital
law_bonus bonus 2 requires event_counter is_the_ai 4 and hidden_resource capital
law_bonus bonus 4 requires event_counter is_the_ai 3
law_bonus bonus 5 requires event_counter is_the_ai 4
law_bonus bonus 2 requires factions { mongols, }
happiness_bonus bonus -10 requires not factions { slave, cumans, mongols, } and not event_counter new_era_begins 1 and hidden_resource cumans
happiness_bonus bonus -8 requires not factions { slave, cumans, mongols, } and event_counter new_era_begins 1 and hidden_resource cumans
; Mongol initial low taxes and terror of rule help them to keep settlements not rioting
happiness_bonus bonus 6 requires factions { mongols, } and not event_counter mongols_invasion 4
happiness_bonus bonus 4 requires factions { mongols, } and event_counter mongols_invasion 4 and not event_counter mongols_invasion 5
happiness_bonus bonus 3 requires factions { mongols, } and event_counter mongols_invasion 5 and not event_counter mongols_invasion 6
happiness_bonus bonus 2 requires factions { mongols, } and event_counter mongols_invasion 6
Much further work is needed, and observations are needed as well, so thanks @Nemesis and @Cherish, for these observations.
Besides, the Mongol script is incomplete, as I've started coding a few months ago but never returned to the issue. It's on the to-do-list, to make it coded in such a way (as I've posted at the time I was busy with it):
------- The Mongols' tweak -------There're 4 goals to attain for the Mongols' tweak:
- give the player starting in their neighborhood (Turks, Abbasids, Cumans) a chance to survive (so that playing those factions is not a hopeless task). To this end, the second wave of attack will be limited in the north or in the south if the player plays a respective faction.
- make them behave historically (be unstoppable at times, go deep in the west, subdue territories, then fade away) but make them part of the game (stochastic: time of arrival, directions of attack, number or armies).
- make them gain territories indeed (what's been seen in the previews, they often linger without taking settlements for dozens of turns).
- fix the problem of public order (rebellions) in the intialy conquered settlements.
- adjust the numbers to the lower number of available troops in the SSHIP (the numbers of Mongols were from SS6.4).
The elements of the tweak:
- increase initial public order in the conquered cities.
- diversify the directions of expansion (not only Khorasan, but also through the steppes to Rus and Europe, and also through the Iranian plateau to Baghdad).
- add additional attacking groups that make expansion more historical.
- lower the experience of the units, and change the composition of the armies.
Programming of the invasions:
- invasions' warn starts a few turns earlier but may also occur later - on average the date will be the same, but there'll be more variability.
- waves of attack:
** 1: 1218-1230 : 2 armies assault Merv, 2 assault Urgench [currently 9 armies + 5 if Merv not taken, what was rather inevitable].
** 2: +3 : 2-3 armies assault Ray or Alamut
** 3: +3 : 2-3 armies assault Tabriz or Baku or Tbilisi or Ani
** 4: +3 : north of the Causacus and east-Black Sea (2 armies near Azaq, 2 near Kiev) [currently none].
** 5: +3 : second wave: 3 armies either at two of those cities: Merv/Urgench/Konjikala (if the player is Cumans), or 3 at Bulgar and Sharukan (if player Abbasids, Turks); or both - if player none of those factions [currently 4 armies at Merv]
- instead of 3 additional groups of 2 armies, there'll be historical invasions:
** invasion of Iraq : if Mongols hold Nishabur/Merv, Ray/Yazd, Isfahan, spawn 2 full-stacks near Baghdad (otherwise spawn them next to Merv)
** invasion of Syria : if Mongols hold Nishabur/Merv, Ray/Yazd, Isfahan, Baghdad, spawn 3 full-stacks near Damascus or Aleppo
** invasion of Anatolia : if Mongols hold Nishabur/Merv, Ray/Yazd, Isfahan, Baghdad, spawn 3 full-stacks near Ceasarea or Sivas
** invasion of Zalesye : check cities against Cuman/slave/Mongol ownership then spawn 2-3 full-stacks at the first: Bulgar, Ryazan, Suzdal.
** invasion of Kiev : check cities against Cuman/slave/Mongol ownership then spawn 2-3 full-stacks at the first: Sharukan, Pereyaslav, Kiev
** invasion of Poland/Hungary : if Mongols hold Kiev/Pereyasla/Sharukan/Azaq/Iasi, check cities against Cuman/slave/Mongol ownership then spawn 3 full-stacks at the first: Halych, Krakow, Esztergom
** invasion of Bulgaria : if Mongols hold Azaq, check cities against Cuman/slave/Mongol ownership then spawn 2 full-stacks at the first: Iasi, Tarnovo, Nis