Stronghold

Stronghold; Museum not yet visible but is left of the rightmost round turret

Stronghold is the primary structure of a Player's Base. Its level determines the level to which all other buildings can be upgraded, i. e. no building may have higher level than the Stronghold. (The dependence works in the other direction as well: higher-level upgrades require having so much Iron at hand that they can't be started before leveling Iron Storages near their maximum currently allowed capacity.)

The Stronghold has storage capacity of 10,000 both Food and Iron, especially important on low levels and needed to be factored in capacity calculations even later (see table below).

The Stronghold also houses the Museum in its right middle part (marked by magenta banners).

The Stronghold may be upgraded to maximum level 25 and gains additional Base space in which to build, along with the corresponding buildings to fill those spaces, at Levels 3, 5, 8, 11 15 and 20. Other Base developments are available at levels 10 and 21 to 25:

Stronghold upgrade space and other developments

Lvl	New Buildings
3	Farm, Mine, Food Storage, Forge
5	Farm, Iron Storage, House, Training Camp
8	Farm, Mine, Iron Storage, Forge
10	Unlocks Barracks*
11	Farm, Food Storage, House, Training Camp
15	Farm, Mine, Iron Storage, Forge
20	Farm, Food Storage, House, Training Camp
21	Unlocks Hunter's Lodge and Advanced Buildings**
22	Allows to convert second advanced Farm, Mine, Food Storage, and Iron Storage
23	Unlocks Alchemy Lab
24	Allows to convert third advanced Farm, Food Storage, and Iron Storage
25	Unlocks Hero Academy

* Stronghold Level 10: Barracks (as well as further unlocked Advanced buildings) are converted from other, already-leveled buildings; see each individual page for information on their own requirements.  Because of said conversion, no new building slots are acquired at Stronghold level 10 or 21.

** Stronghold Level 21:  Unlocks the following Advanced Buildings: Advanced Farm, Advanced Mine, Advanced Food Storage, Advanced Iron Storage, Advanced House and Hunter's Lodge, which can all be further upgraded to level 10.

Stronghold Upgrade Costs

Lvl	Iron Cost	Time	Iron/hr	Minimal Iron Storage prerequisites
2	4,900	1m	294k	(none – basic Stronghold 10k)
3	12,300	10m	73,800	Iron Storage Level 1 (2.5k)
4	26,500	55m	~28,909	2x Level 2 (9k)
5	47,700	2h 15m	21,200	2x Level 3 (22k)
6	75,600	4h 25m	~17,117	3x Level 3 (22k)
7	113k	7h 35m	~17,538	3x Level 4 (36.8k)
8	162k	11h 45m	~13,787	3x Level 5 (53.9k)
9	228k	17h 5m	~13,346	as above, new 4th to Level 6 (72.7k)
10	301k	23h 35m	~12,763	3x Level 6 (72.7k) + Level 7 (93.1k)
11	388k	1d 7h	~5,791	3x Level 7 (93.1k) + Level 8 (115k)
12	489k	1d 17h	~6,351	3x Level 8 (115k) + Level 9 (138k)
13	619k	2d 3h	~5,033	Level 9 (138k) + 3x Level 10 (162k)
14	753k	2d 15h	~5,578	4x Level 11 (188k)
15	903k	3d 4h	~4,908	2x Level 12 (214k) + 2x Level 13 (242k)
16	1,073k	3d 19h	~5,391	as above, new 5th to Lvl 10 (162k)
17	1,260k	4d 12h	5,000	3x Level 13 (242k) + 2x Level 14 (270k)
18	1,490k	5d 6h	~4,869	5x Level 15 (299k)
19	1,719k	6d 2h	~4,776	Level 16 (329k) + 4x Level 17 (360k)
20	1,969k	7d	~4,688	5x Level 18 (392k)
21	2,108k	8d 1h	~4,383	5x Level 19 (424k)
22	2,367k	9d 4h	~4,351	4x Lvl20 (457k) + Advanced Lvl 5 (537k)
23	2,439k	10d 9h	~4,005	as above, 2nd new Adv to Lvl 4 (523k)
24	2,545k	11d 15h	~3,770	3x Lvl20 (457k), Adv8 (579k), Adv9 (594k)
25	2,591k	12d 19h	~3,506	as above, 3rd new Adv to Lvl 2 (497k)