is it possible to have a horizontal main menu, and, if applicable, a vertical submenu on the lefthand side? Main menu should only show pages on highest level, submenu should show the subpages underneath the chosen page on the main level.
I’m not too sure what you mean, but have you looked at the Secondary Navigation add-on? It will allow you to insert a vertical menu into one of the sidebars while keeping the main navigation up top.
I’ve found that add-on.
But is it possible to show only the subpages of one top-level page in this menu?
For example:
Top-level pages : A B C D
Sublevel : B1 B2 -> these only need to show up in subnavigation when clicking on top-level page B
Sublevel : C1 C2 C3 C4 -> these only need to show up in subnavigation when clicking on top-level page C