Iron Cellar Door
Press & hold Activate to lock it or set a combination. Repair with Forged Iron and a construction tool using the secondary action. Upgrade to a Steel Cellar Door using a crafted Steel Cellar Door.
Upgrade path
Hit a placed block with a stone axe, claw hammer or nail gun while carrying the material of the next step: 6 hits per upgrade.
Crafting
Craft at Backpack · makes 1
Forged Iron× 10
Harvest drops
Iron× 40
Taken apart with a harvesting tool (axe, pickaxe, shovel), before perks and tool bonuses.
When it collapses
Falling, it leaves 
Scrap Metal Pile (75% of the time) where it lands.
Where to find (9 POIs)
Aranea Caverns (tier 4), Don Vittorio Spagetto Residence (tier 4), Jim & Tim's Bullets and Booze (tier 3), Ranger Station Echo (tier 2), The Shady Swine (tier 2), Roberts Tack Shack (tier 1), Tracey Family Shelter (tier 1), Best Pals Shelter, Bunker Buildsite #5.
Points of interest built with this block (any of its variants), the highest tiers first, from the game's prefab files.