Trying to find this old movie I saw many many years ago. In B+W with men in togas and Roman style buildings and several shots of rockets fired out of a huge vertical gun.
Might this be Things to Come (1936)...?
The story of a century: a decades-long second World War leaves plague and anarchy, then a rational state rebuilds civilization and attempts space travel.
There are scenes with ancient Roman-style buildings and people in togas, and the third act features a craft being launched toward the Moon from a giant space gun.
The only movies I remember with manned rockets being shot out of vertical cannons are From the Earth to the Moon (1958) and Things to Come (1936).
The 1958 black and white movie The Brain Eaters has an object which looks like it could be a spaceship that could have been fired from a giant cannon, but its oriign is somewhat different. It was set in the USA in the 1950s and so nobody wears toga-like garments.
And I am only certain about the cannon in Things to Come (1936). From the Earth to the Moon (1958) was based on the 1865 Jules Verne novel of the same title. In the novel a giant cannon was constructed to shoot the capsule to the Moon. Since I haven't seen the movie From the Earth to the Moon (1958) for a very long time and do remember that it changed a lot of details from the novel, I can't be certain the spaceship was shot from a rocket in the movie.
This review: https://classicscifi.blogspot.com/2009/03/from-earth-to-moon.html says the spaceship is shot from a giant cannon.
From the Earth to the Moon (1958) is in color, but might have been seen on a black and white television set. I have first seen a lot of movies and tv shows on black and white television screens and assumed they were in black and white, but later discovered they were in color. Things to Come (1936) is black and white. https://en.wikipedia.org/wiki/Things_to_Come
From the Earth to the Moon is set in the USA during the Grant administration of 1869-1877, and so the characters all wear clothing which doesn't look at all like togas, unlike the future people in Things to Come (1936).
Everything considered, the movie you are asking about is probably Things to Come (1936).