how did the franks tried to make their hiding place secure

From the text:

"The hiding place was located in Father's office building. That's a little hard for

outsiders to understand, so I'll explain. Father didn't have a lot of people

working in his office, just Mr. Kugler, Mr. Kleiman, Miep and a twenty-three-year old typist named Bep Voskuijl, all of whom were informed of our coming. Mr. Voskuijl, Bep's father, works in the warehouse, along with two assistants, none of whom were told anything."

"We started off immediately the first day sewing curtains. Actually, you can hardly call them that, since they're nothing but scraps of fabric, varying greatly in shape, quality and pattern, which Father and I stitched crookedly together with unskilled fingers. These works of art were tacked to the windows, where they'll stay until we come out of hiding."

"We've forbidden Margot to cough at night, even though she has a bad cold, and are giving her large doses of codeine."

"Of course, we can't ever look out the window or go outside. And we have to be quiet so the people downstairs can't hear us."

"Mr. Kleiman and Miep, and Bep Voskuijl too, have helped us so much. We've already canned loads of rhubarb, strawberries and cherries..."

"Because so many houses are being searched for hidden bicycles, Mr. Kugler thought it would be better to have a bookcase built in front of the entrance to our hiding place. It swings out on its hinges and opens like a door. Mr. Voskuijl did the carpentry work. (Mr. Voskuijl has been told that the seven of us are in hiding, and he's been most helpful.)"