I was watching a documentary on Ancient Egypt and they talked about door locking devices that were left in tombs. The devices were not for locking doors in the tomb, but for locking doors in the real world. Some historians hypothesized that these devices may have lead to formalizing privacy. Wondering if anyone had any information on these ancient locks?
The general principles of lock design haven't changed that much since ancient times. Even the modern pin tumbler lock, which dates back to the 1840's has precedent in some locks found in ancient times. The Young Folk's Book of Invention from 1925 devotes a whole chapter to locks and keys and you can see in the first diagram a lock found in the ancient Palace of Khorsabad in Mesopotamia. You can see that the pin mechanism is not that different that what you would find in the lock in your front door.
The website RomanLocks.com has an extensive collection of locks and keys from ancient roman times. You'll notice that the section on padlocks is far more extensive than the section on door locks. Presumably the vast majority of locks being used were padlocks as there would have been more versatile in their application. Roman locks weren't that different than many modern locks, though I don't think the Romans were using any pin tumbler locks. Most of them are variations on warded locks, which are one of the simplest and oldest lock designs. Most modern warded locks tend to be the tiny locks that you use to lock a cheap chest or jewely box or the one that comes on a diary. This style lock is not difficult at all to pick though I don't have any resources about ancient lock-picking styles, presumably because most of what we know about ancient locks are from the locks themselves. A skeleton key is one that is specifically made (or filed down from an existing key) that can be used open many warded locks, which is one of the known common ways that lockpicking was undertaken in ancient times. Modern pin locks made skeleton keys obsolete (which is why you tend to hear about skeleton keys in stories and video games that take place far in the past ;).
Follow-up question; how effective where they? What were lockpicking like?
I love locks! To start off with, I found a fantastic blog post about the origin of locks, and I definitely couldn't give a better overview, so I recommend reading that!
With the assumption that the above article will cover Mesopotamian and Egyptian locks satisfactorily, I'd like to bring up their precursors, the simple-yet-elegant door seal!
The earliest evidence we have of some form of lock are the door seals from the ancient Near East, specifically at an Iranian site known as Tall-E Bakun A, dating to around 4000 BCE, where door seals were found in five of the buildings excavated. Abbas Alizadeh in The Origins of State Organizations in Prehistoric Highland Fars, Southern Iran: Excavations at Tall-e Bakun notes that all instances of seals in ancient Persia are found in secular buildings, in contrast to developments in Mesopotamia where there's a concentration of such administrative implements in temple complexes. You can see a drawn reconstruction of one of these sealed doors here. They're essentially a cord tied around a peg in the door-jamb, sealed with a lump of cylindrical clay. The seal would be an impression made by a specific "stamp" and was often baked to harden it. It's clearly doubtful how effective these would've been at actually preventing entry, but they did ensure that for as long as the seal was intact, no one without the appropriate measures to re-seal it would have had access to the room. The rooms guarded in such a way were usually storage rooms in the back of buildings which likely contained various goods probably intended for some form of official distribution (perhaps not any goods that were very valuable, as the seal wouldn't provide much deterrence from theft). Alizadeh says that the fact that the seals in Tall-e Bakun are found in large quantities in clusters, all baked, is evidence that they would've been part of an organized record-keeping or accounting system. For a very informative break-down of the distribution of these seals and the authors' interpretations, see pages 85 - 90 of the book cited above. It's very interesting, and available for free download!
These early seals in Persia are essentially the same sort of technology that were widespread throughout Anatolia and Mesopotamia from the 5th and 4th millennia BCE onward, albeit with regional cultural differentiation.
Fiandra, "The Use of Clay Sealings in Administrative Functions From the Fifth to the First Millenium BC in the orient, Nubia, Egypt, and the Aegean: Similarities and Differences" is another excellent source on storerooms, distribution, storage and the like, and is a bit more focused and readable than Alizadeh's broad excavation analysis. Lots of details.
Just for a visual reference, here are a few examples of cylinder seals throughout the ages, the most common type of seal in Mesopotamia.
Anyways! To finally address what OP was interested in, how did folks lock their doors? The answer for the average ancient Mesopotamian seems to be that either they didn't, or they did so with a simple mechanical latch. More important administratively-regulated rooms could be kept fast with devices of widely varying complexity, from tied cords and latches sealed with imprinted clay, to large and secure locking mechanisms employing pins and keys.
