In terms of the OP/not OP discussion, perhaps a side-by-side comparison to boneyard/graveyard is in order?
Boneyard:
Cost: 3
Trigger: creature dies
Creature stats: 1/1
Graveyard:
Cost: 5
Trigger: creature dies
Creature stats: 2/2
Forge:
Cost: 5
Trigger: a pillar is played
Creature stats: 2/1
Mithril Forge:
Cost: 5
Trigger: a pillar is played
Creature stats: 3/2
Looked at side-by-side, I think that yes, it is OP compared to graveyard/boneyard. Playing a pillar is a more common trigger than the death of a creature, and one that you can cause yourself without slowing your deck down at all, even if your opponent is playing a pillarless deck.
Particularly comparing the upgraded versions, forge is better in almost every way. The cost is the same, but it gives you better creatures, and I'm pretty sure it would trigger more often, and against a larger variety of opponents.
Steel Golem is an
creature, so I think
fits the golems a little better than
, but I like the forge itself as a
card. (parts coming together to form a whole -> gravity)
I really like the concept, and I'd like to see more swarmer decks that don't depend on fractal, but I think this needs a bit of a nerf.