Traditionally the Red Fort is used in a defensive role, or occasionally for comical rapid battlefield support if playing as Americans. These are perfectly viable uses, but the Red Fort can also be used much more offensively by making use of its rapid healing.
Garrisoned land units have a base healing rate of 1 HP per 20 frames, which is 0.75 HP per second. With Healthcare technologies (researched at the Granary), this increases to 1 per 15 (1/s), 1 per 10 (1.5/s), and finally 1 per 5 (3/s).
The Red Fort claims to heal units inside at +500% speed (i.e., 600% speed). However, in practice it heals at more than 600% speed at most tech levels:
|Normal healing rate
|Red Fort healing rate
|Red Fort bonus rate
|1 HP per 20 frames
|1 HP per 3 frames
|1 HP per 15 frames
|1 HP per 2 frames
|1 HP per 10 frames
|1 HP per 1 frame
|1 HP per 5 frames
|1 HP per 1 frame
The specific numbers here are high enough to not especially matter for this tactic, I just wanted to make note of the above quirk.
Note that healing from green Patriots will be — at best1 — the same as the base healing rate, i.e., 0.75 HP per second.
In short: if you build a Red Fort as close to the frontline as possible, you can use it to make a tactical retreat-to-heal, heal up unreasonably quickly with the Red Fort, and then return with healthy troops to continue the fight while your opponent’s army is still injured. Due to the non-linear scaling of army strength, being forced to fight at even 3/4 HP against a full HP army can have a substantial effect on the outcome of the fight.
The further your Red Fort is from the fight, the less viable this tactic will be of course, and the more it’ll be eroded by the defender researching faster healing. As their rate of healing increases, your window of opportunity conversely decreases, and your longer-than-the-defender’s travel time can end up taking as long as the defender’s now-reduced healing time, leaving you with no advantage in total-time-to-redeploy.
If you can get your troops to retreat, heal, and return all in a minute or less, you should reliably have a meaningful health advantage over an opponent with only either no-tech/Herbal Lore or Patriot healing. Once your opponent has Medicine, a full minute can be a little dicey, as whether this works will depend on how hurt the defender’s troops were and how quickly they were garrisoned to heal up. Success will be situational depending on each side’s travel time and healing rate. Most of the time you can get a decent advantage, but I wouldn’t bet a game’s outcome on success unless most of the various factors are in the attacker’s favor.
Once Pharmaceuticals is in play, you should expect this tactic to no longer work regularly at all, as garrisoned units will heal quickly enough that you can’t reasonably make a round trip to the Red Fort faster than the defender can heal to full. It will still work sometimes because not everyone will diligently garrison their troops, but a strategy that relies on your opponent making a mistake probably shouldn’t be the first thing you reach for.
Forced March is essential to save as much time as possible, and since you need to use it on both legs of the round trip to/from the Red Fort, you’ll either need to be using a red Patriot (with at least Forage researched at the Smelter), or have more than one Patriot/General on hand.