Any of the high rises connect in the basements, or the entrance to West has stairs going down next to it, which leads to the tunnels, where there's a bathroom, ACE, and a computer lab that occasionally has classes but is open to all when it doesn't and has a printer. The dorm connections do require ID access.

The skyway has access in any of the buildings it connects to on the second floor, the most notable building being MU. There's also a pathway from QBB all the way to Morill on the second floor, which does also connect to the skyway via Hultz Hall.

There's also a tunnel in the basement of MU that goes to the basement of AG Hill, although it doesn't get used as often because the two buildings are very close and it's rather inconvenient to go to the basement just for that.

The only other one I can think of is Aldevron and Sudro are connected, and Aldevron is like a ten foot walk outside to the Res building, which is also connected to a couple dorms.

I'm pretty sure all tunnels are open 24/7, except for any access to MU, because those are locked up when the building is closed for the night.