i'm new so i don't know if this has been stated before but i'll try my luck
forge
the forge is a 4x4 upgrading facility unlocked at th6 that is similar to the lab but different in a few key features including
- all upgrades cost gold (including the forge)
- upgrades buildings not troops
- adds additional features to buildings to change defensive and offensive gameplay
fortify (th6)
turrets/towers can be incorporated into walls (they are classifed as walls afterwards)
defenceive positions (th7/9)
town hall and clan catle gain 1/2 archer/s on the roof
emergency troops (th8)
when an archer or wizard tower dies they drop that many troops
training camp (th7)
army camps can now offer gold bought alternatives for any troops that they hold
examples:
barbarian
sword and shield
-damage
chance to ignore single target damage
wizard
fire rune
this requires a small change to the current wizard
-attack speed
DoT to all surrounding structures and units
ice rune
-damage
slows target by 20%. stackable
lightning rune
-AoE
chain attack hitting up to 7 targets (1st=100% damage, 2ndx2=50%, 3rdx4=25%)
earth rune
-damage
each attack pushes back troops
Valkyrie
Mjölnir
-attack speed
chain attack dealing 50% damage to a second target within a 3x3 circle of the original

