Galders Tower uses?

Can Galders Tower be summoned on an enemy? Are there any restrictions in the casting/location of the summoning? If you were to keep an enemy monologing or otherwise engaged for 10 minutes, and

conjure a two-story tower made of stone, wood, or similar suitably sturdy materials

10 feet above the enemy, how much damage would it deal? would they be able to get out of the way? I don’t know how a creature could make a save to avoid a tower up to

100 square feet.

that they are in the middle of. furthermore, could you cast the spell to trap a creature inside of it? it doesn’t specify that you need to put an entrance on it, nor that it specifies a type of material, only that it

two-story tower made of stone, wood, or similar suitably sturdy materials

would that allow materials such as Mythril or iron?