The old "Stepped Door" designs had a purpose in the old days of cowboys and into the early 20th century.
These safes were hand crafted to fit together very tightly, with very close tolerance door gaps. Door and Body frames are constructed from layer upon layer of tightly fitted steel plates, usually constructed from a really badass metal known as Mangalloy, or Hadfield Steel after the inventor. Mangalloy is a highly Manganese enriched carbon steel (13-15%) that presented one of the hardest metals known at the time (mid-1800's). This steel is highly abrasion resistant, and when you try to bend or impose high impact, it work-hardens to extraordinary levels.It was developed for use in earth moving equipment and other abusive environments.
These unique properties made it highly desirable to make high security safes that were Dynamite and Nitro-Glycerine blast resistant boxes. The close tolerance door layer fitting prevented an adequate volume of nitro-glycerine from collecting in the door closure gaps to minimize the blast effectiveness. For liquid explosives to work best, they needed to be injected into the door gaps. If the gaps were very tight, very little nitro could be placed in that zone. This, coupled with the extraordinary metallurgical properties of Mangalloy was about as good as it could get.
This technique later gave way to the easier-to-manufacture Round Door Safes (one example is the "Cannon Ball") and big walk-thru Bank Vault Doors, where a turned machined surface was much easier to provide the ultra tight fit necessary to provide nitro joint infusion resistance. This is why you see the older mammoth vault doors were made to be round, not square. The old Cannon Ball safes were a monolithic one-piece cast ball of Mangalloy, usually placed on a pedestal, and fitted with a round door. These things were nearly impossible to open until modern high speed power drills and carbide drill bits came along.
Today, high-tech plastic and nitro explosives are so closely controlled that there is little concern for them being used any more by thieves. The explosion resistant UL TXTL safe ratings are all but forgotten in the market today, and I don't think anyone has a current active TXTL listing any more. When explosives are (very rarely) used today, there is a different approach that is far more effective, and very dangerous to use.
AMSEC used Mangalloy, trade name Manganal, in TL-30 doors into the late 80's. Alas, the cost became prohibitive and thicker AR400 and AR500 plates served as well in the UL test labs, so Manganal was abandoned going into the newly implemented 1994 retest cycle and doesn't appear any more in safes today. AMSEC was founded on the Round Door safe in 1945 by Gene Halls under the name Star Safe and Alarm, and his designs are largely still produced today as the Star Lift-Out Door. AMSEC TL15 and TL30 Round Doors were produced until the UL retest program forced us to limit the number of safe listings due to the extraordinary cost of testing.
So, now you know how the stepped door came to be, and where it lead in safe evolution.