Cages are not just a box with bars. They are our bird's refuge, safe spot, their address when we are away. It's not a barrier, it's their environment. It has a huge unrelenting effect on them. So, let's look at cages like our parrots do.

Next time you have an opportunity to stick your head in a large parrot cage, do it. Stick your head in as far as you can, or just remove the bottom tray and grate and get in it if it's big enough. Look up, look down, look around. The confinement is real. The bars mean something. Do you feel that? There are two ways you can make this confinement feel; comforting or restrictive. There is the truth of your cage purchase.

8 Parts of a Cage to Consider

1. Color. Don't choose the color for you, choose it for them. Stick with light colors. Visually for a parrot, they are less constrictive and present as they look through to their outside view.
2. Doors. If a bird has to duck, twist, move, or make themselves smaller to get in the door while on your hand; the door is too small and the cage inappropriate. That door signifies a transition moment. It is either comfortable, or it is uncomfortable and disconcerting. Cage doors and the items near doors are core issues for birds with problems exiting or entering their cage. Multiple doors can be a problem as they create an issue with toy placement. Multiple door cages have a specific purpose for smaller birds and breeding but offer nothing to larger parrots.
3. Locks. A good cage with good maintenance can last a lifetime. When you are looking at cages consider the locks and how they work. You will be using those a lot. Find out if there are replacement parts for the food and door locks. If there are, order two replacements on the spot. With the medium to big birds, it is only a matter of time before they figure them out, take them apart and loose the bits and pieces when you aren't looking. Butters, our macaw, removed all ten lock mechanisms on both macaw cages over 2 days. From inside and outside of the cage. Smart girl. The replacement parts were $10 a piece when I bought the cages 4 years ago. At time of her mechanical wonderment, they had gone up to $20. Smart girl, expensive fix. Make sure there are replacement parts available, even for the little ones. Things wear out under normal use, too. And down the road when you need them, they may no longer be available. Cage manufacturers are not like car manufacturers.
4. Cage shape. Square, rectangle, or multi-facet corner designs, as long as they are not round. Round cages, no matter the size, are a mental health hazard. There is no other way to say it. If you are in a temporary situation and have a round cage here's a trick: Attach a straight dowel rod perch, take an old T Shirt and run one arm sleeve through and then the other. Now you have a curtain that makes a 90-degree wall. Attach your parrot's favorite perch near this so they have a wall to perch next to, it's a great temporary fix.
5. Cage size. A cage should provide more than enough space to jump, short flight, flap, hang upside down, climb, forage, and play. When cage shopping, consider the toys and how much space will be left after hanging them and inserting perches of different sorts. A cage looks huge when empty.  A cage looks small filled properly. Think air space. Consider all the elements of the cage and you'll see in the end; you really cannot purchase a cage that is too big.
6. Bar Gauge and bar spacing width. Generally, the bigger the cage, the bigger the gauge and width. Manufacturers assume you'll be placing certain types of birds in these cages. Bars are horizontal or vertical as well. Consider your bird's head size. Make sure the bar spacing does not accommodate them getting their head through the space. Because they will try. Consider the vertical/horizontal bars themselves. Horizontal bars allow more climbing and more options for toy and perch placement. Vertical bars on two of four walls are a fair split for options as well. A cage with all four walls vertical will pose annoying problems for work ability in toy and perch layout.
7. Perches. When you find a cage that meets all the above considerations and bring it home, remove all the dowel rod perches immediately and replace them with rope perches, tree branch perches, and nail care perches. Perches are a bird's shoes. The more varied a parrot's perches the healthier their feet are going to stay. Consider your shoes. How many different pairs do you have? Why? I have what I like to call "slipper" perches in every bird's cage. It's that one perch that's cotton rope they all seem to prefer for a nap. And those cotton rope perches can be washed in the washing machine or dishwasher. Just lay them out in the sun for a good baking to dry off.
8. Cage placement in your home. Again, consider what it is like to look out of the cage. Don't tuck the cage into a corner near a pass-through door. Every time someone comes in, they will be surprising your parrot with their arrival. A very stressful place for a parrot. Window view is always appreciated as well. Consider your windows, are they drafty? Does the glass build up heat? The best cage location provides lots of line of sight for your parrot to keep an eye on things, an outdoor view for interest and light, and clear understanding of sounds and footsteps. Keep your cage out of the line of air vents as well.  

One last point; the exterior and top of the cage.

Play top, dome, open top; the options are available to create a play area near and on their cage. Consider how you would like them to view their cage when they are not in the cage. Creating an outside environment that is enjoyable, safe, interesting and personal to their likes will create a cage that never feels foreign, it always feels like home.

Parrots hang upside down, fly anywhere in any manner and are fearless in body position and seeing things. It is a challenge as a human who generally sees the world on a X-Y axis to assimilate their view of their world. But it helps to try.