I have a need to create a custom font for my own use in a printing software.
The output should be tower style, where text reads from top to bottom.

The software itself can't do this automatically, only rotate a textual object 90 degrees right.

Hence, I need to rotate each glyph 90 degrees LEFT.

The idea is: object => [ABC], rotated with custom font should be printing
A
B
C

I tried using FontForge and rotate glyph, but it gives all kinds of warnings when I generate font.

I know too little about font metrics. Is this doable at all ?

Thanks for any input.

http://www.dafont.com/vertigo-brk.font

Menhir: that is exactly the orientation. But I need something that is more "Arial" like. Not possible to find for me.
So that's reason for my question on how to rotate.

I think the font "Aachen Vertical" is exactly what you need but is a commercial font.

Yes, it is very long to do.
You are in the wrong forum.