Fortress and Stronghold are, in Century Games' own words, two types of buildings for the same gameplay — Fortresses simply grant better rewards. Everything below is from the publisher's English help centre, because this is one event where the win condition is genuinely counter-intuitive and guessing costs you a whole cycle.
The win condition is a stopwatch, not a kill count
Once the battle starts you can rally the building or attack it alone. To take it you must be in control for more than 30 minutes. And here is the clause that decides most Fortress battles: if no alliance manages to occupy it for 30 minutes, then whoever is in control at the end of 2 hours becomes the occupier.
Read that twice, because it inverts the obvious strategy. Throwing everything in at minute one is how alliances lose Fortresses. The building is won by whoever can still be standing on it near the 2-hour mark — which means holding troops back, keeping your Infirmary from overflowing, and timing your heaviest rallies for the back half of the window rather than the front.
Registration: who, and how many
Only members above R4 from an alliance with 20 or more members are eligible to sign up. If your R4 says the button is greyed out, count your roster first.
The alliance leader confirms the Fortress or Stronghold to attack and selects three different times; the server then decides the final battle time from the selections of every alliance registered for that building. So your preferred time is a vote, not a booking — and if you are in a small alliance competing with a large one for the same Fortress, expect the time to land on the big alliance's preference.
How many buildings can you contest
Each alliance can sign up for one Fortress and one Stronghold per phase. First-timers get one building only. If you occupied something in the previous phase you are automatically re-registered for that building's battle, and you can still register for one new Stronghold or Fortress on top. Century Games' own example: hold Fortress No. 5 this phase, and next phase you are auto-entered for No. 5 plus whatever new target you pick.
The exception worth diarising: if the current phase is the final phase of a season, the automatic re-registration does not carry into the first phase of the next season. Alliances lose buildings they thought were safe every season on exactly this clause. Put a reminder in your alliance chat.
Troops actually die here
This is not a simulated battlefield. Casualties are real, so raise your Infirmary capacity before the window rather than after, and know your own limit: per the Combat FAQ, in Sunfire Castle battles casualties go to the Infirmary until it is full and only then start dying. A full Infirmary during a long Fortress fight is how a "successful" defence turns into a rebuild.
One myth to retire: Fortress battles are not the only alliance content where troops die. Century Games' Facility article states that battles at Lv. 4 Facilities also result in lost troops, while lower-level Facilities only produce severely injured ones.
Before you press register
- Confirm 20+ members, and confirm your R4 knows the three-time-slot mechanic is a preference, not a guarantee.
- Check whether you are auto-registered from last phase — and whether this is the season's final phase.
- Expand the Infirmary. Then check the rewards through the building's own View panel, which Century Games points to explicitly, because reward tables differ between a new target and one you already hold.
Sources: Fortress battles — rules and the 30-minute / 2-hour condition · Why can't I sign up for Fortress battle · One Fortress and one Stronghold per phase · Combat FAQ — Infirmary and casualties · Facilities — Lv. 4 battles kill troops
Every rule above is Century Games' own wording from the official English help centre. Anything we added from play experience is labelled in the text.