@D11M
There is no ferocity limit on the second army and you may attack as often as you want. You do not get to chose a specific player to attack, it's all random but within certain limits (Easy, Medium, Difficult).
@Bored - the costs of upgrading the fortress doubles with each level and the time it takes for the upgrade adds 8hrs/level (lvl(n-1) time + 8 hrs).
So, level 3 fortress (Lvl (n-1)ATK+500; Lvl(n-1)DEF+1500):
Gold - 25.000 (lvl(n-1)x2)
Wood - 10.000 (lvl(n-1)x2)
Stone - 10.000 (lvl(n-1)x2)
Water - 10.000 (lvl(n-1)x2)
Weat - 12.500 (lvl(n-1)x2)
You can get several elementals of different type during attacking another fortress (provided you are successful). You also get some points for overall rankings, a few honor points and some blueprints to be used by the Defense Engineer (new building available, max. 2 of them) to produce traps/explosives/saws to help defend your own fortress.
The fortress fight system is a mix between PvP and Guild Arena: you do not lose anything (points, gold...) when attacked and lost (PvP) yet you can't chose your target upon attacking (Guild Arena).
Cheers,
st0rm