Locksmith was a real profession. Locksmiths were not just repairing and selling locks, as now, but actually making them. And a wealthy Renaissance or Medieval household had a lot of locks in it. Material goods and food were expensive, so lots of things would be locked away: there'd be locking cabinets holding bread, locking chests and wardrobes holding clothes, locking coffers holding money and jewels. And, in control of it all, the housekeeper and the lady of the house, with bunches of keys hanging in clips at their belts, jangling authoritatively around the halls.

The locks were warded locks- the wards were sometimes simple, sometimes elaborate obstructions inside the lock that prevented any other lever than the fitted key from sliding open the bolt. The shaft of the key could also be a fitted tube, instead of a solid shaft. An elaborate example of the warded lock of 16th c. Italy ( or perhaps Nuremberg, which was famous for metal work) here will give you the general idea.

The safes were ironbound chests. If they were simpler ones, there would be strap hinges and hasps on the outside, and padlocks to secure them, like this one . Those were weak points- a thief could batter off the locks or hinges, or just dig through the exposed wood of the sides and claw out something from within. So, the highest level of security would be something like this, with not only heavy iron binding, but sheet iron beneath that, to shield the wood from chiseling. The bolts would be internal and impossible to batter, the hinges, (like the hinges on a bank vault door now) were exposed but because of the internal bolts, battering off the hinges would get the thief nowhere. And on this example, there's even a false keyhole, luring the thief into spending hours trying to pick a non-existent lock.

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Vincent J. M. Eras: Locks and Keys Through the Ages