I wish my first flock would have been so talented. I lost all but 2 to predators. We know for sure we had a bobcat in the area. We have Hawks which we are sure got some of them. I had 3 hens and a beautiful rooster and 4 juveniles. I was left with just the roo and one juvenile. I rehomed them. That was tough! Wild Bill was my baby. Anyway, we can't free range so put up a 10x10x6 kennel. We built a door for them to go in and out. Lock them up at night. Eventually another will be on the front of it. They can go front to back both covered. They're doing fine, but they'll do better with the added pen in front. I tried free range it failed miserably. There are free range chickens down the road. Don't know what losses they have but my flock is so small losing one is one too many.
I'm tempted to make a chicken tunnel. Seen pics on line of them. I'd like to have them running in one when I'm home. They'd love it.
Your city neighborhood has Bobcats? They're nasty. I'm so sorry to hear about your losses. My DD lives in the suburb city but her property butts up against a natural local preserve for hiking and trails - it has attracted Coyotes, Lynx, Bobcats, once a Bear, and a couple years ago a Cougar! Not to mention smaller pests like Rattlesnakes, Skunks, Coons, Possums, Wild Rabbits, Deer, loose Dogs, Hawks, etc. They've caught photos of several wildlife critters on their night camera.
I'm further away from the Mountain behind us and have been fortunate not to have critters like Skunks or Snakes - but that doesn't mean there isn't danger from someone's loose Python or Boa pet or the city night critters like Coons and Possums. We all need to assess our property for local predators and build accordingly. The key to our 4 hens' survival in our little cottage backyard has been several scattered shelters, doghouses, and popup canopy and we plan to add stickery evergreen, roses, and berry bushes along the fence line - all for the girls to hide/snooze under during the daytime. We have a giant flock of neighborhood Crows that are wonderful at leaving our chickens alone and chasing off the Cooper's Hawks (chicken hawk). I never cared for Crows until I learned how valuable they were at chasing off predatory birds. The Crows don't even bother our vegetable beds.
I've seen chicken tunnel photos and videos too. Sounds like a neat idea if one can be constructed sturdy enough to keep out digging canines like Coyotes, Foxes, Dogs, etc. We are getting rid of our 60-y/o chainlink fence and replacing with a 6-ft block wall with added privacy fencing to make it even taller. Won't keep out the Hawks but will make it difficult for digging critters to enter!
I live in a very rural small town. We don't have a fenced yard. Plus, we are on a corner lot with the other sides complete over grown. None clears their lots beside or behind us ever. Deer come out into are yard at night but so doe other pesky night time varmets. We are only about 30 miles from Galveston as the crows. I went a long time with no trouble so I started thinking my flock could free range in the day time, oh well. My new flock is doing great. They look so healthy. Their feathers are soft and shiny. I'm not going to feel bad about confining them. I do feel bad it's taking longer to get the other pen up. Life's drama has gotten in the way but it will get done. Sounds like you have a great setup. Lots of cool little hiding places :-D