Defender was part of a new upstart video game division at Williams, who were known for their pinball machines. (there was a Pong variant they put out previously, but I don't think it was mass produced)
since they had no previous "assets" to work with on the video game side, they basically started swiping parts off the pinball line to complete the game. the original silver doors were lifted directly from pinball. the sound board as well -- it and the speech add-on that was on Sinistar are the exact same boards you would find in a System 7 Black Knight. (and actually, there was a second rectangular sound board that was from the previous generation pinball games.)
the MPU (CPU) was allegedly supposed to be a pinball design as well, not sure if it preceded System 7 or if it was intended for the next revision, but the video side wound up claiming it. if you compare the reset circuits between the video MPU and the System 7 MPU, you'll see some peculiar similarities. the video MPU obviously needing video output and what was then a ton of ram was the major difference.
the boards are mostly arranged as they would be in one of the Williams pinball machines. the power supply looks similar but is used for much different purposes from their pinball counterparts.
the reason for the black door switch however was because at the height of Defender's production they were apparently churning out 2000-3000 games a week, and because they were robbing pinball of their parts, it was slowing pinball production. Defender ultimately was the bigger money maker, by virtue of its production run spanning about 128,000 units, so it kind of received priority. the concession was to change the coin door so they had the silver doors for pinball. in the later part of its run, they started using Coin Controls doors instead, which I think were the same as the ones Atari was using, albeit it had the pinball style test panel attached to it.
so when I say Defender was rooted in Williams pinball, this is what I mean.
