Solution for WololoKingdoms RMS and Scenarios (No modding required)
#const SGTWR 885
effect_amount UPGRADE_UNIT SGTWR HORSE 0
effect_amount SET_ATTRIBUTE SGTWR ATTR_TRAITS 3
effect_amount ADD_ATTRIBUTE SGTWR ATTR_GARRISON_CAPACITY 1
according to John the Late the unit needs to have the appropriate tasks in order to use those unit traits, which is stored in one byte, with each bit corresponding to some trait.
The first bit is the flag for garrisoning. The line effect_amount SET_ATTRIBUTE SGTWR ATTR_TRAITS 3 sets the first and second bit.
But the reason why you can't just upgrade a horse into a siege tower and garrison it is because it's missing the task for that
according to jineapple unit id 885 is the old siege tower from a forgotten empires campaign, in wololokingdoms, that shoots arrows but can't jump over walls. I suggest using that.
If you want more garrisonable units, maybe start thinking about modding.
USA
katsup or mustard[This message has been edited by local boi (edited 05-15-2019 @ 04:09 AM).]