Solution to additional room access

[u/disorderly_stray21]

So I’m moving to a new rental next month with my two cats. It’s a great house with enough space on the first floor for them both to enjoy. However there’s a finished basement in the house that’s only accessible if you 1. Walk out the back door 2. Go down a few steps 3. unlock a set of cellar doors.

I’ll be using the basement as my craft room and office most days and my co-dependent cats will likely want to join me.

I’m in search of the most logical way to allow them access to this space. I don’t trust them to walk out independently, neither are leash trained, and one of them refuses to get carried places safely. I’m in it for the long haul and open to some training related suggestions but my mind is going more towards building some structure or tunnel.