I'm a bit confused about how Aid works (e.g., Assault Aid). I just created my first unit that provides Assault Aid (Polybian Legionary). I see that it has a promotion called Assault Aid I, and the tool-tip indicates that promotion grants "+3% Strength." I have a few questions:
1. Does that mean that all units stacked with my Polybian Legionary have their Strength increased by 3%?
2. Is that 3% Strength increase only for limited purposes (e.g., only attacking, or only attacking cities) or does it count for all purposes (e.g., attacking and defending under all circumstances)?
3. Does the 3% Strength bonus stack with bonuses from other Assault Aid units? For example, if I have two Polybian Legionaries in the stack, does the stack now get +6% Strength instead of +3%?
4. Can I upgrade this unit from Assault Aid I to Assault Aid II? If so, how? That promotion doesn't seem to be available even though the unit has XP to spend.
(Short answers: (1): No, only those with equal or lesser strength. (2): All purposes. (3): Yes. (4): No.)
I'll try to explain the aid system for you in a nutshell. The aid "promotion" is a temporary promotion units grant to other units in their square. Spending XP has no bearing on it. Different types of units grant different types of aid. Melee units grant assault aid, which is just a flat strength increase which works both attacking and defending. Archery units grant first strikes and city defense, siege units grant city assault bonus, etc...
Each aid has three types, "I", "II" and "III", progressively stronger. A unit can receive only one of these types and which it gets depends on its relative strength versus the total strength of all the units granting that particular aid in its square. The aid gets upgraded to type II if the strength of the unit(s) granting the aid is at least twice that of the receiving unit (*), and to III if it is at least three times that. I'll illustrate with some examples. (EDIT: I explained this in a very confusing way earlier by having 'type' refer to two different things. Sorry about that. Edited to be more clear.)
Say you have an archer unit of STR 4 in a square with a bunch of polybian legionaries, also STR 4.
If you have unit of each, the archer receives Assault Aid I and the legionary receives Ranged Aid I.
If you have one archer, and two legionaries, the archer receives Assault Aid II and both the legionaries receive Ranged Aid I, and Assault Aid I from the other legionary.
If you have one archer and three legionaries, the archer receives Assault Aid III and all legionaries receive Ranged Aid I, and Assault Aid II from the other two legionaries.
If you have one archer and four (or more) legionaries, the archer receives Assault Aid III, and all legionaries receive Ranged Aid I, and Assault Aid III.
Adding more legionaries does not change the situation beyond this point.
Hope this helps.
(*) I'm actually not sure how the rounding of the relative strengths works in determining the aid. Must the total strength of the aiding units be at least twice so the aid upgrades from I to II (always round down) or is it enough that it is at least 1.5 (rounding normally), or is it enough if it exceeds it even by a little bit (always round up)? I also am not sure does it count the maximum strength, or does being wounded affect it.