Can all the doors and windows of the house be made of steel?
Yes, if Saturn is exalted in your chart. The same thing that harms one person can benefit another.
Related questions
Our small house has an iron door outside and a wooden door inside. Which is the main door, and what if the inner door is on a wrong pada?
The wooden door is the main door. Iron is acceptable in the West, and an iron door in the boundary wall is acceptable. If the inner main door is on an unfavourable pada, correct it with the direction-wise bandhanwars and symbols taught in this class plus some door power. Moving it to S4 suits only if the male head of the house has Kark, Makar or Kanya rashi, because Vedic vastu judges this from the male head of the house alone, not from anyone else such as the wife.
Can doors be opened anywhere by vastu, or must the birth chart be checked?
Check the birth chart. If the Sun in the chart is debilitated and you forcibly open an E3 or E4 door, it will be wrong and not appropriate at all.
If the Moon is debilitated, is a North-West entrance bad?
Yes, not just bad but very bad; the North-West entrance will have to be closed.
Is choosing the door related to astrology?
Yes. With the person's astrology as the base, the door is chosen according to whom it will suit. The class covers three ways, by rashi, by exalted and debilitated planets in the birth chart, and by mulank, and the birth chart carries the most weight.
My house entry is in the SSW and my 10th house has Sun, Mercury, Moon and Ketu with an exalted Venus; can I move the entry to the South-East?
If you open the South, open it in the good South pada (S4 as heard), and if you open the West, in the good West pada (W4 as heard). The present South-West entry should be corrected with door power.
Should window shutters open outward or inward?
Outward. The door brings energy into the house, while the window is there to throw out the negativity created inside, so its shutters should open outward.