The first two games were top-down 2D stealth games. The third was a pseudo-3D FPS. Most people picked up the series at the third installment, and the third installment is what made it into history books as the First Popular FPS, ignoring earlier FPSes which were Not Popular.
To be clear, Castle Wolfenstein and its sequel were written by Silas Warner and published by Muse Software, which went out of business in 1987 after going through Chapter 7 bankruptcy. Because the trademark was lapsed, Id was able to use the name for their game. But it wasn't a continuation of the original games, more like a reboot by another team and publisher.
Random fun fact: the only reason they could use the name is because the original creator let the trademark expire, thinking it had no commercial future
