Not easy to make. Under Study! (as @ 2009)
Cannot be made exactly into a font. But an approximation is possible by creating a pseudo-shadow effects within the limitation of a two-tone glyph:either black or white.
Here is an example:
Letter A:
However one limitation of this approach will be the resolution of the thin lines on a monitor, especially at small size fonts.
Update as @ 25/10/2010
However, since writing the above note about two years ago, I have "cracked the code" so to speak, and found that the Capitalis Monumentalis, of which the best example is that appearing on the Trajan Column in Rome, is based on the modulus of the golden ratio. I still cannot create the three dimensional effect on the font because of the two-tone glyph limitation, but the modulus, or proportions of the script, are now well understood. A font is now under development and will be presented as soon as I have completed it. I consider this a major finding and will be writing a paper on this same page to explain the geometry behind this script.
Example of work-in-progress of a flat Trajan Font: each the following letters has been cross-checked for proportions and modulus with the letters on the Trajan Column:
Further update as @ August 2011
It seems that I may have found a way of creating realistic engraved text, not as a font, that is certainly not feasible, but as sculpted text using Autocad. This is now being tested.