I have our robo in a Savic Mickey 2XL and it's great with the narrow bars but I would stick with tank style if you have cats. I would also suggest you keep the robo in a room where cats can't go because it will be stressful for a robo smelling cats in the vicinity. They are the smallest species of hamster and the most fearful - understandable really.
So - I originally had a Kerry Terrarium from Zooplus for our robo - didn't quite have room for another 100cm cage. It didn't work out for me really. As I prefer cages and front entrance but also I felt it could do to be a bit bigger. It was ok but ..... The Savic cage isn't much bigger - but feels a lot bigger and I can fit a lot more in.
I personally would go with a maxi duna multy if getting a tank style for a robo - the 100cm one. So much scope for setting it up with lots of variety and levels for a robo. The lid has bar spacing that is less than 1cm (about 8mm I think). Although be aware that even a tank style will have a lid and cats will try and paw it - so really is best to keep the cage in a cat-free room if at all possible. The cats can free-roam elsewhere
It isn't cheap though - about £80. There is also the Skyline Marrakesch tank on Zooplus (you can get it cheaper on Amazon in different coloured wood) which is a lovely tank about 95cm and has a good lid with tiny mesh. But it is over £100.
You could also make a bin cage - if you're up for the diy and a good sized bin would be the largest Really Useful box which is about 80cm by 60cm. Then you can choose the size of mesh to use or even fit an old cage door to the front of it.
Or there's the detolf if you have space!
https://www.google.co.uk/search?q=ha...w=1138&bih=503
This is the maxi duna multy
https://www.amazon.co.uk/Ferplast-Ma...axi+duna+multy
And this is the Savic Mickey 2XL - a robo would not fit between the bars. Although there is one area on the front door where it bends where the bar spacing is 1cm between two of the bars. That bit could be blocked off if necessary.
Amazon is the only place that sells it and the price goes up and down now and then - usually about £80 to £86 and currently way expensive at £99.
There are pros and cons. With a tank with a lid - you may leave the lid off or forget to put it back on which is then no protection against a cat.
With a cage like the savic mickey 2xl you have more interaction with the large front door and are less likely to forget to close a big door like that, which fastens tightly closed.
Main thing is to keep the cats away.
https://www.amazon.co.uk/Savic-Micke...vic+mickey+2xl
Ultimately though - if I had to have a tank style I wouldn't get anything smaller than a 100cm and probably the maxi duna - because the wider it is, the easier it is to use the top access and have your arm going in at an angle. Plus you can make it a wonderful habitat.
Robos don't like to feel too exposed so need lots of things to hide under and run through so you would need a free-standing platform. A tank with straight sides is easier to fit a platform in.
This is the Marrakesch (not cheaper on Amazon at the moment!) The wood is wipe clean - waterproofed with something.
https://www.zooplus.co.uk/shop/small...es_wood/437277
Our robo absolutely loves his labyrinth house from rodipet. Dark inside - rooms - hammies do seem to love labyrinth houses. So a cheaper cage option and a labyrinth house could be one way to go. You can get a Barney cage cheaper than any of those listed above (about £50), which is a 100cm cage with 7mm bar spacing and a good shelf. The main issue with that is the height/falls risks so could be tricky to set up and you'd end up spending a lot on shelves and contents to make it safe possibly. Also the top door may be a bit of a risk. Never shuts tight closed and I use the top door a lot so it's a pain pegging it down if needed (ours has a syrian in).
There are very few cages with narrow bar spacing. Cypher has a lovely one - the Marchioro Kevin - slightly bigger than my Savic Mickey 2XL - but they aren't made any more and very hard to find second hand.
Cheapest option is probably a bin cage with the largest Really Useful Box which costs about £39.
Next cheapest probably the detolf, then the maxi duna multy, then the Savic 2XL, then the Marrakesch.
Oh and there's also this! You'd probably need to paint the wood inside with plastikote petsafe waterproofing paint though (the base is protected already).
https://www.hamster-homes.com/shop/x...ome-90cmx50cm/
Of all of the tank styles the maxi duna is easier to move/handle when necessary and fitting a platform shouldn't be too difficult.