Well, on the C64 version of BTI, the third level of Harkyn's Castle is not teleport-shielded. I suspect this to be a programming oversight, considering that level 2 is shielded.
Not shielded is nice. I used to, way back, have a good strong guy in slot 1 and my ATEAM guys in 2-6. Then I'd APAR them up, hit the berserkers and MIBL the ever-loving blazes out of the Legions of Harkyn. Great level-up system that way.
I would think it is open because of Kylearan's tower. Given that there was no "save-anywhere" feature, it would be very annoying to have to hack your way through all of Harkyn's, face the Beserkers, then Tarjan, then navigate your way through Kylearan's.