(More of a memory dump related to Stephen's answer)
At a time when ICs were of low complexity (compared today), could you actually see each transistor on the silicon and reverse engineer it?
Just try it yourself. Take some 1980s TTL, like a 7400 - I'm sure you find some on old boards - and crack it open. Usually it separates well from the plastic. Put it below a children's microscope and you will see the structure for each of the four gates. With some basic knowledge about semiconductors, you'll be able to draw the schematics free hand.
It's what I did to my first pocket calculator at the age of 13. Of course, I didn't have the knowledge to really turn what I saw (it was a calculator chip, not just a 7400) into a useful drawing, but I still remember my father's face, showing a mixture of being mad and amazed at the same time, when I showed him the drawings and he realized that I utterly destroyed the calculator he has spend ~50 Mark (*1) on a few days before. Essentially it foreshadowed my future of taking everything apart :))
Slicing up a 1976-1980 8-bit CPU is not really challenging. Structure sizes were about 5-10 µm, so not even a microscope was needed. A good camera with a decent lens and average film will do it.
Let's take for example the well-known 6502. It was manufactured in an 8 µm process with a chip area of roughly 4x4 mm. The needed film quality is defined by 'lines' - that is, pairs of consecutive lines that still can be distinguished. 8 µm wide details on a 4000 µm means we need at least 2000 lines over the width of the photograph taken. With 35 mm film, this means 2000/35 = ~60 lines/mm. An average ASA 100 colour film will deliver anywhere between 60 to 150 lines/mm.
So even the cheapest hobby film could be used in reverse-engineering a 6502. This is especially true as we talk analogue film, so no pixel artefacts (*2). Of course, this is the upper limit to start, still making it hard to decide some structures. So more lines are better. B&W films of similar (low) quality and price deliver 300-500 lines/mm, while good go past 1000. More than enough to give a crisp clear view of every detail (*3).
Bottom line: in 1980, a consumer class camera with a good lens and cheap film was all that was needed to dissect any CPU at the time ... well, yes, and a lot of time at hand to understand what was shown :))
In fact, one can still do so with the latest chips except the problem is that nowadays, a picture, even a very good one, wouldn't do it alone. The 6502 had 5 layers and basically all of them can be identified right away. Nowadays chips are made of more than a dozen layers, many of them connective ones, covering the underlying transistors a great deal. Today it needs careful scraping of of layers to get a look inside, much like archaeologists digging out a settlement. But still doable, as now fine tools like lasers can be used to peel each layer. Heck, there is even a guy in Berlin(?) doing this for fun with today's chips, like Apple's latest ARM cores.
*1 - 50 Mark in 1974 may equal about 100 Euros nowadays. Doesn't sound much, but it was quite a lot money to us (you know, ye goode olde time when we had no shoes and had to walk to school in snow and rain, uphill in both direction). Back then, filling a large car (Mercedes) was less than 20 Mark.
*2 - To be correct, analogue film also shows granulation, as the photo sensitive area is made up of discrete particles. But unlike a CCD they are randomly placed, resulting in kind of a ... well ... analogue resolution. Important for this task is that due to the random placement none of the usual digital artefacts will be created. The picture will simply start to blur.
*3 - Superior material can deliver up to 10,000 lines. That's enough to cover any modern CPU with a single shot. Of course, at that point optics does start working a bit differently, but that's another story.