Champion's Retreat and Reno Kits
Champion's Retreat
Champ retreat can be won in special events and are a good fast unit (through Modern) or heavy unit (Postmodern and up) to have in any age for GvG. A lot of fighters use Champ + 7 rogue almost exclusively for fighting in GvG, because artillery and ranged units tend to get placed a lot as defense armies (artillery and ranged are deadly to heavies, which most people prefer to use with rogues, but no problem for Champs!). Champs become especially good from Postmodern onward when they are heavy units instead of fast units, but I was also partial the the Progressive era champs, they do well against everything except for the PE tanks.
Champs are as good as a regular horse unit in Iron thru Industrial ages, but take less space/population than cavalry type barracks, that's one of their main advantages. 1 champ for the chivalry bonus can be good to include in DAs for GvG sectors, or 1 in your city defense.
Champs are good to mix in 1 with any defense army or city defense, as their Chivalry ability grants extra boosts to the remaining units when they are killed in battle. It makes your city or defense army in GvG just a bit tougher to beat when there is at least 1 champ present. Also Champs tend to charge into battle and do real damage with the AI fight simulation, whereas 'regular' fast units tend to hang toward the back and waste time on the battlefield in the opening moves.
Avoid using more than 1 champ in city or DAs, the chivalry bonus does not stack with multiple champs (this is also true of color guard and drummers - more than one does not help) and a champ has the highest point value to kill of any unit in each age; people who farm for points put 8 champs in a sector on purpose to kill them for max point value - and it is a weak DA to everything except light units.
Reno Kits - Collecting Champs from Every Age
It is a strategy many players use to have 1 or 2 Champ Retreats in the city or saved in inventory, and use them with RENO KITS (Renovation kit can be won in special events and is a precious commodity! or you could also use the new One-Up kits) to upgrade Champs to a new Age as we progress in the game. This gives a big advantage, if you can Upgrade a Champ retreat you have units to compete in a new age immediately upon going up, and assuming you also have Alcatraz, you retain a nice army of unattached Champs from the previous age, which you can keep for a long time if you use them carefully.
Using reno kits with Champ Retreats, and Alcatraz of course, you can build a nice army of Champs in every Age as you go up through the ages. With events and opportunities to win Champ barracks being more plentiful in the game, it would be wise to save a champ retreat from each age of the game using Store Building, and you can always place them in your city again any time you need a small footprint building (the retreat is only 3x3 and builds instantly) to generate troops quickly in any age.
Fighting with Champs - Effectiveness
You do have to tell horses and other fast units to stay back on the first turn when using them to attack, or the fast units will run up and get skewered, and then all your rogues die in one hit and you lose the battle. As long as you tell them to stay on the first turn, champs and other fast units work well paired with rogues.
The higher your attack/def boost gets the more the game becomes about fast units (as opposed to heavies which hold up better) because what matters is reaching the enemy before they get a chance to hit you. I probably did 90% of my fighting in lower ages with 1 fast/champ 7 rogue, or 1 heavy 7 rogue - one of the two of those combos kills just about anything you'll face in GE, GvG, or Hood. The province map (quest line/continent map) can be another story as rogues are not targeted by the computer AI there.
CE Champs are useless in GvG DAs and siege armies, 8 assault tanks is much harder to beat in GvG. When I fight champs I barely take injuries, if I fight assault tanks I lose 1 or 2 units every time. However, with a very high city defense boost, Champs attack is the same as Assault Tanks but their defense is significantly higher. Assault Tanks base defense is 90 while CE Champs is 125. With a High Boost 8 CE Champs is sturdier than 8 Assault Tanks in your City Defense! And 2 CE champs and some rogues can be a good attack army for auto battles once your attack/defense is high enough; high attack boost champs take less losses when attacking than regular tank units, and don't need to be told to stay back on the first turn like anti-air units do.
Champs are good in Postmodern era and below, in CE they have limited uses, and TE is the last age I thought champs were really great; I have kept a Tomorrow champ retreat in my city for years. In FE and above, the champs are not great because the regular heavy unit outshines them: FE (champ outshined by hover tanks, which is probably the best unit in the whole game,) Arctic (champ outshined by battle fortress), Oceanic (crabs and octopods both better) and even Virtual champs (Ronin superior), Mars and SAAB champs are all next to useless except to give max points to anyone who is attacking. Champs are the most valuable unit for points in every age, so you will see them farmed and killed for points on the GvG maps.