Yeah, I knew that buildings could not calc. That code I put is for a tactical regen spell used in combat which I am curious if it will work.

I was curious if there was possibly another way to do it with cities. As in using a multiplier. For example I have seen resources multiplied like this:

<GameModifier>
<ModType>ResourceMultiplier</ModType>
<Attribute>Gold</Attribute>
<Value>1.2</Value>
<PerTurn>1</PerTurn>
<Provides>+20% Gildar</Provides>
<UpgradeComparisonID>2</UpgradeComparisonID>
</GameModifier>

Regen looks like this:

<GameModifier>
<ModType>City</ModType>
<Attribute>AdjustDefendingUnit</Attribute>
<StrVal>UnitStat_HealthRegen</StrVal>
<Value>5</Value>
<Operator>+</Operator>
<PerTurn>1</PerTurn>
<Provides>Heals 5 Hit Points per Week to all stationed units</Provides>
</GameModifier>

So maybe if a UnitStat_HealthRegenMultiplier could be made, not sure.

So if that could be made somehow then maybe:

<GameModifier>
<ModType>City</ModType>
<Attribute>AdjustDefendingUnit</Attribute>
<StrVal>UnitStat_HealthRegenMultiplier</StrVal>

<Value>1.25</Value>

<PerTurn>1</PerTurn>

<Provides>Heals 25% health per week</Provides>
</GameModifier>

But yeah, I don't think that is possible heh.

I am trying to get it to regen as a percentage of max hp. That was a silly mistake, heh. I am playing around with a bunch of things, hmm.

If it doesn't, you can just make an ability to auto cast the spell, assign it to a custom unit, have the building summon that unit as a defender. Then have the ability cast a second spell that kills the units.

I tried to do it as an enchantment but it didn't work, blah.

No idea.

I have no idea if this will work, but try to put a code to pull the units max health and multiply it by 0.1. Pretty straight forward, and it entirely depends on the logic of the original programmers, but it might work.

<GameModifier>
<ModType>City</ModType>
<Attribute>AdjustDefendingUnit</Attribute>
<StrVal>Unit_GetHPCurrent</StrVal>
<Multiplier>1.4</Multiplier>
<PerTurn>1</PerTurn>

</GameModifier>

<GameModifier>
<ModType>City</ModType>
<Attribute>AdjustDefendingUnit</Attribute>
<StrVal>Unit_GetHPCurrent</StrVal>
<Multiplier>.4</Multiplier>
<PerTurn>1</PerTurn> 
</GameModifier>

Neither works. I have tried other things too but I just can't get it to work.

<GameModifier>
<ModType>City</ModType>
<Attribute>AdjustDefendingUnit</Attribute>
<StrVal>UnitStat_HitPoints</StrVal>
<Multiplier>.4</Multiplier>
<PerTurn>1</PerTurn>

</GameModifier>

Tried other things too

Honestly not sure if those are possible. I wouldn't know how to code it off the top of my head

You could perhaps track the issues which give the city hp or which are based on issues which give hp and give a regeneration bonus based on each of them?  This would not be perfect, but it might be acceptable?

However, note that high level regeneration can be frustrating to deal with.

For example... one of the things that increases the total hp of a city is its population -- you get units based on population, but also population is what gives you city levels, so you could approximate some of this gameplay effect by giving each subsequent city level a certain amount of regeneration.