You conjure a two-story tower made of stone, wood, or similar suitably sturdy materials.
I think most DMs would qualify iron as too study to be "suitably similar" as wood.
As for dropping it on a creature. Nothing is specified, so it is up to DM discretion.
The DMG provides a bit of guidance for improvising damage:
10d10 Crushed by compacting walls, hit by whirling steel blades, wading through a lava stream
18d10 Being submerged in lava, being hit by a crashing flying fortress
If its avoidable you could look at Tasha's rules on falling:
If a creature falls into the space of a second creature and neither of them is Tiny, the second creature must succeed on a DC 15 Dexterity saving throw or be impacted by the falling creature, and any damage resulting from the fall is divided evenly between them.
And for a similar example from a published adventure, Storm King's Thunder includes a portable boulder you can expand on command and drop on your foes.
A creature can use an action to shrink the stone to the size of a 6-inch-diameter orb weighing 25 pounds, or enlarge the boulder to its normal size (15-foot diameter) and weight (12 tons). Anything the enlarged boulder falls on takes 55 (10d10) bludgeoning damage. A creature can avoid taking this damage by tumbling out of the way with a successful DC 15 Dexterity saving throw. Once the reducing or enlarging effect is used, this property can’t be used again for 24 hours.
For another comparison.. Instant Fortress magical item if deployed with helpless creatures in the way offers..
a DC 15 Dexterity saving throw, taking 10d10 bludgeoning damage on a failed save,
I am starting to notice a pattern of arbitrarily large but not absolutely massive objects impacting you offering a DC 15 Dex save or dealing 10d10 damage. Whether save avoids half or not is up to your DM.