Cat "tree" placement?

I have 2 1-year old sibling cats, both fixed. They like play time and rest time, like most cats. I am about to buy them a 6-ft tall cat "tree" with cubby holes and rest pads. I am wondering whether to place it near a balcony window, so they can see outside more, or in a more "chill" place. ... show more I have 2 1-year old sibling cats, both fixed. They like play time and rest time, like most cats. I am about to buy them a 6-ft tall cat "tree" with cubby holes and rest pads. I am wondering whether to place it near a balcony window, so they can see outside more, or in a more "chill" place. Looking for advice from others who have similar structures for their cat(s), and where the preferred spots seem to be.

They are strictly indoor cats. I live in Bangkok; if they went outside they'd be squished in 15 minutes. They like to look at birds. There are no other pets in the house, and they groom and love each other. The purpose of the "tree" is for stimulation, not to get away from anything, The vigorously play daily.
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