Unlike in AoE or RoR monks can heal your troops from a certain distance. Units now also automatically heal when they're garrisoned inside buildings. Teutonic monks receive a healing bonus and can heal your troops from twice as far as all other civs' monks. When you're allied with a Byzantine player your monks also heal 3 times faster than usual. Besides all that, one of the Vikings' unique units - the berserk and it's upgrade the elite berserk - automatically heal themselves when injured, even during battle.

As you can see AoK healing is a lot more complex than just a technology to speed up the healing process so that's why I decided to give it a little more attention.

(*): This is the time needed in seconds to heal 100 HPs from one unit.

more test results

• Monastery upgrades or age advances don't affect healing in any way.
  
• Monks heal a lot faster than buildings but keep in mind that monks can only heal 1 unit at a time while buildings heal all garrisoned units simultaneously. A theoretical example: To heal 20 elite Teutonic knights from 0 to 100 HP (2000 HP in total) a single monk needs 800 seconds to heal them while a single castle can do it in 500 seconds!
  
• You can add multiple monks to speed up the total healing time. Adding multiple healers doesn't work downgradable like adding multiple builders to a building site does.
  
• Berserks always benefith their unique healing abilities, no matter whether they are in battle, being garrisoned or being healed by monks. For example an injured berserk in a castle heals at a rate of 0.533 HPs / second which is the sum of the castle healing rate and the berserk healing rate.
• Monks can heal allied units but they don't start healing them automatically, even when the monk is idle and within range. It is
  possible though to manually give your monk the command to heal your ally's units.
  
• It is possible to garrison your units in your ally's buildings. Not only do they add extra firepower to the building but they also start healing automatically.
  

Test procedure

monk healing rates

To test the monk healing rates I started from a random map in the scenario editor. I added a monk, an elite war elephant, an enemy castle and a watch tower. I used the castle to quickly reduce the elephant's HPs and the watch tower for the finer work . When the elephant had only 2 HPs left I brought in my monk and started healing. The monk needed 3:49 minutes to restore the elephant's HPs from 2 back to 600. So it's obvious that healing 600 HPs takes exactly 4:00 minutes or 240 seconds. The monk healing rate is then 2.5 HPs / second.

To check whether age advances or monastery upgrades had any affect I did the test again with post-imperial Teutons (who have access to all monastery upgrades). The result was the same so there was no influence.

To test the Byzantine team bonus I added a third player (Byzantine of course) to the scenario and allied with him. Now it took the monk only 1:20 minutes to heal the elephant 598 HPs. Byzantines indeed add triple healing speeds to the team, the healing rate for a monk when allied with a Byzantine player is 7.5 HPs / second.

building healing rates

To test the building healing rates I always used Teutonic knights (100 HP) instead of elite war elephants (600 HP) because you can only garrison foot units in all the buildings and the elite Teutonic knight is the one with the most HPs. I also used an enemy watch tower here to "prepare" my Teutonic knights for the test. In 16 minutes (960 seconds) I could restore 96 HPs of a Teutonic knight in the TC. This means a TC heals 1 HP every 10 seconds or has a healing rate of 0.1 HPs / second.

I did the test again and healed the Teutonic knight in a watch tower. The test results were the same. I retried in a bombard tower and again the tests were the same. Only in the castle the test results were different. When I ungarrisoned my injured Teutonic knight after 7 minutes (420 seconds) its HPs were restored from 2 to 86. Thus a castle heals twice as fast as a TC or tower and the healing rate is then 0.2 HPs / second.

I retried healing in a castle with the Byzantine still allied to me while I was Teutons in post-imperial but the healing rate stayed the same. That means the Byzantine bonus doesn't affect healing in buildings (as expected) nor do age advances or monastery technologies.

I also tried healing a 2 HP Teutonic knight with 2 monks and it took exactly
20 seconds before the knight's health was fully restored. So for 2 monks
the healing rate is 5 HPs / second. This means that you can add multiple
monks to speed up the total healing time and that it doesn't work downgradable
like it does when you add multiple builders to a building site.

berserk and elite berserk healing rates

I did the same basic test as with the elephants but now with berserks in castle age. But now instead of healing the unit with a monk I let it auto-regenerating and timed the process. It took the berserk 2:30 minutes to heal itself from 5 to 55 HPs. The healing rate for berserks then is 0.333 HPs / second.

I did the test again in post-imperial age with elite berserks. Now it took 2:52 minutes to restore the HPs from 15 to 72. After some maths you can see there is no difference between berserk and elite berserk healing rates (also 0.333 HPs / second here).

Just out of curiousity I put an injured berserk for 1 minute in a castle. During that time the unit's HPs were restored from 10 to 42.
Again, after some math you can see the healing rate is 0.533 which is exactly the sum of the berserk healing rate (0.333) and the castle healing rate (0.2). So berserks keep their special healing bonus also when garrisoned inside buildings. I didn't test healing a berserk with a monk but I assume there the healing rate would be 2.833 HPs / second which would be the sum of the berserk healing rate (0.333 Hps / second) and the monk healing rate (2.5 HPs / second). So no matter under what circomstances (garrisoned, being healed, in combat, ...) the berserk is, he always keeps its unique healing bonus.

ranged healing

To test ranged healing I placed a special wall segment on a random map in the scenario editor to easily count the healing distance. I used an enemy bombard tower to injure my elite war elephant and placed it at one end of the wall, a monk on the other end of the wall. Then I clicked my monk and ordered him to heal the elephant. When the monk stopped moving and started healing I counted the tiles in between. The normal monk healing range was 4 tiles. I repeated the test with Teutons and there the monk healing range was 8 tiles, like expected. Changing settings to post-imperial didn't affect the healing distance at all. The first picture shows the test with
Persians (range 4), the second one with Teutons (range 8).

healing allies

The first test was done very quick. I placed an allied TC, tower and castle near my base and an enemy tower a little further away. I used the tower to injure my 3 villagers and placed them in the allied TC, tower and castle. All 3 buildings restored my villagers' HPs so units also heal in allied structures.

The second test required a little more setup. I started with 3 civs and allied with the 3rd one. I gave the 3rd payer a war elephant and boxed it in with its own walls. Next to the walls I placed an enemy watch tower. A little further I placed a trebuchet and a monk from my own player colour. Then somewhere else I placed a second enemy watch tower and an elephant of my own. Then I ran the scenario. When my elephant was half injured I withdrew him out of the range of the enemy tower. When the allied elephant only had about 20% of his HPs left I unpacked my trebuchet and destroyed the enemy tower. Then I moved my elephant close to the allies
elephant. Then my monk approached from the other side. My ally's elephant was the closest to the monk and the most injured but my monk started healing my elephant first anyway. When finished, he didn't automatically start healing the allied elephant but went idle. When I manually ordered the monk to heal the allied elephant the healing process started.

Conclusions

It can take very long to heal units inside buildings and therefore I believe the 100G cost for a monk is always worth the expense if you want to heal your troops. Besides, you might want to get a monk anyway to garrison some relics or maybe to convert a few units. Nevertheless, if you decide to heal inside buildings anyway then do it inside a castle if possible. Units inside castles heal twice as fast as they do inside TCs or towers.

Berserks heal themselves faster than other units do inside castles. A Berserk garrisoned inside a castle benefiths from both healing rates and restores his HPs at a rate of approximately 1 HP each 2 seconds which is not so bad at all. And since they are created in a castle it should be not much of a problem to let them regenerate there as well .

I was hoping that with the ranged healing in AoK idle monks would automatically start healing allied units but obviously they don't.

Then I also wanted to add a section here about healing priorities but since it requires a lot of testing I will save it for a future article. With healing priorities I mean testing which units a monk starts healing first when you send him inside a heterogene group of injured units.