I drilled holes in the window sills and then screwed the perches down.  Didn't depend on the double sided tape.  

Pros: Good size, cats love them and can sit and look out windows for hours.  Like these much better than those with wires to hold them in place.
Cons: Only con is that the plastic supports do NOT stay in place and fall off.  We had them pushed in and slid into a locked position but they don't stay.  However, the perches, being screwed down are very stable and I haven't had any probems.