There are several ways to heal units, each suitable for particular conditions depending on the extent of the injury, your ability to pay for full recovery, available builders and serviceman nearby, etc.

All mobile units have an Auto-heal ability that uses the unit’s own restorative functions. Damaged units recover 3-5% of their current Attack Strength and Health every turn, which makes the process of regaining full strength free but very lengthy. The Auto-heal is 2x faster when the unit is inside it’s “builder” (City, Shipyard or Stable).

Self-healing can be imagined as the basic medical care soldiers got in military camps; the time required to restore their full strength would be directly linked to the severity of their injuries, note however, that although this action is also free, the unit is immobilized during the process and the ability of the unit to defend itself if attacked, is drastically reduced (-50% and -60% Defense Penalty). The unit regains 10% of its current Attack Strength and Health per turn, so any serious injuries will take a long time to heal.

Recovering can be imagined as replenishing the ranks with new recruits or conscripts. This means Recover can be only executed when a unit is close to a city from which the new recruits can be drawn. This way you can get men and their basic equipment and armaments, but no proper training; thus, the unit recovers its strength, but its Experience points are reduced. Note that its Loyalty and Morale may be affected if the cities from which the replacements are drawn, have low Loyalty and/or General Morale. This action requires resources to attract men to join the army. The unit regains 20% of its default Attack Strength and Health per turn.

The Heal inside action can be executed only when a unit is moved into a city or building with the correct specialization. Using the Heal action restores the full strength of the unit much faster and in fact, costs less.

Generally, the more injured or damaged the unit is, the better off it will be, if it sent to the nearest city to Heal inside. Naturally there may be situations when this isn’t possible, in which case you can use Recover and Self-healing.

The Medics improvement is an option that enables a unit to recover its strength automatically over several turns without any additional costs. It increases Attack Strength and Health by 25% of its current strength every turn. It can be gained through being trained, as an improvement or it can be temporarily received in the healing temple Asklepeion or by visiting Naiads (if Myths & Legends feature is enabled in the game).

Units can be also healed without incurring further costs in cities with the Hospital improvement.