The biggest leap forward in software development for me was the shift to a development station consisting of a keyboard and a CRT (it was a custom unit produced by Intel in the days before the PC was in existance). It seems to me they cost several thousand dollars. As best I recall the first ones I used were large dark blue boxes that included a slot for a 8″ floppy disk. The marvel was that one could make changes to the assembly (mnemonic) code and reassemble without having to repeat all the steps. If the assembler detected errors you could see them at once and it was a much simpler matter to make a fix or two and immediately re-assemble to see if everything was OK. On top of that the magnetic storage was so much easier and quicker than punched tape.
Emulators were not available then so one of my debugging strategies was to write in a temporary break point and then see if the program flow reached that point. If there was an output to an LED in the hardware it was easy to do something to the light so I could see how far the program got. There were also single-step modes, but that was very time consuming and would mess up I/O to things that were time critical.
I am amazed to think back over the hours of laborious debugging that I did with the limited set of development resources that were available. Many times the process entailed poring over the instructions for hours looking for logic flaws that made the program assemble OK but not do what I wanted. I can remember feeling like I needed some cooling system for my brain so it wouldn’t burn out. The one positive aspect of this process was that one became intimately acquainted with machine operation and assembly language.