Rescue: Was It ...

. . . a fort the kids were building? You know, with gates and turrets and moats and solid stone walls and stuff?

Or a tunnel that was a kind of play-fort?

It's the former, right?

So, why just call it a fort when it was really a tunnel?

It's not wrong that you refer to the fort in some way; just make sure the context is correct. One of you found a great way to say it: the boys were digging a tunnel to create a play fort.