Retrieve the Smelter from the Understreets Overlord Guide

Walk down the stairs to the left from the entrance. Let reds take care of the fire (#1) and head down, to the lower level of Understreets - Understreets (2). Loot the first room and walk further ahead killing bugs and zombies and pillaging the area. Eventually, you'll enter a room with a wheel (#2) so turn it and go through the door that's just opened.

Kill your enemies there and walk ahead - to mana pit (#1). There are spawning pits, too, a few steps further so replenish your minions if you need and continue your way to watch zombie starring in a scene. Kill those bastards there (starting with the boss), loot the room and turn the wheel up on the platform (#2).

Walk through the door you've open and follow along the corridor - you'll come to more spawning pits and even more zombies. Kill them all and use the mechanism in the back (#1). Go to the door and there you have it - the next smelter for your tower (#2). Exterminate the zombies again - starting with the boss like before - and send 10 minions to carry the object to the waypoint. With the new smelter you acquire new armory options, so go to the tower and check it out!