Yes, I only placed caesar's as a simple example, there's several "old school" cyphers that can be used as extra "layers", all the owner needs to know, is in which order he used them (I'd be VERY BAD to forget that! but a memory aid can be added as a "sidenote" in the paper, with another, simpler code).

My personal favorite is the ADfGVX, I wrote about it a long time ago here: The most famous Field Cipher: ADfGVX - And its important Role during WWI - Steemit