Many a besieged city has surrendered upon hearing the distinctive rumble of the juggernaut.

Juggernaut

Juggernaut attacks each combat if able.
Juggernaut can't be blocked by Walls.

"Urza's machines have a splendid habit of excavating themselves."
—Rona, disciple of Gix

Dominaria Remastered

Jumpstart 2022

We had taken refuge in a small cave, thinking the entrance was too narrow for it to follow. To our horror, its gigantic head smashed into the mountainside, ripping itself a new entrance.

30th Anniversary Edition

If Juggernaut can't attack for any reason (such as being tapped or having come under your control that turn), then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.