How to heal heroes and armies

• By default, you'll heal a very slow 5% per turn.
• Standing on a friendly city boosts this to 10% (garrisoning has the same effect; it doesn't stack).
• Watchtowers give you 20%, and this rises as you improve them to Lookout Towers and Remotuscope Factories

This may not be enough, though, so you can also:

• Ally yourself with the Sisters of Mercy. Each of their villages gives +5% per turn, which can add up to a huge bonus (healing 1/3 of your hp per turn with 5 or 6 such villages).
• Get a hero (necrophages or sisters of mercy) with a skill that boosts % healing per turn).
• Win the Legendary Deed that gives you Intelligence Corps for a flat +20% per turn.
• Build the Medical Center and/or Inoculation Station buildings in your city (for roughly 25% total).

These would all add up to a ton of regen; really the idea is that 5% is slow enough to really cramp strategic movements, because it's a painfully long 20-turn process to heal your troops if they get badly hurt - but by the time you're up to 25% or so, troops can fully heal in a mere 3-4 turns.

So the purpose of having all these different ways to heal your troops is that they cover each other if one of them becomes unavailable - if you can't (or strategically don't want to) get the necrophage hero, you can grab a different source of regen to get by. If the enemy grabs all the sisters-of-mercy villages, you can invest in the technologies - if you have the sisters, you can completely skip those techs/buildings and get ahead other ways.

You could stack everything for a wacky 60-70% regen per turn, but it's really not much better than 25% - by the time you reach 25% or so, you're healing in a few turns, and your meaningful military power is less about weathering repeated assaults, as it is about your units not getting killed during an individual assault - as the old saying goes, you can't heal being dead.

This is also because usually you're only up against a few armies; you're not withstanding some constant onslaught - if you survive to heal at all, you've almost always killed many/all of the enemy fighters in their army, and usually they only have a few armies in any given war (and will sue for peace after those die).