Is a teleportation circle’s shimmering portal always open?

There is some disagreement regarding the status of a shimmering portal for a permanent teleportation circle.

My understanding is that the permanent circle has a permanently open shimmering portal.

The contesting opinion is that circle still requires a wizard to use a spell slot to open a portal in the circle.

To me their interpretation defeats the purpose of a permanent circle.

If a random squirrel discovered a permanent teleportation circle, would they see an open portal that they could enter?