How do you solid windows in GZDoom (Doom In Hexen format)?

Use linedef flag "Block Everything".

And use tag "impasable" as well. Just for sure

47 minutes ago, Marlamir said:

And use tag "impasable" as well. Just for sure

Block everything implies impassable, although I prefer to do flags separately since block everything also blocks attacks and depending on the type of window, certain attacks would go through it.

I know about that, however i prefer impassable tag in most places anyway.

The only benefit of setting "Impassable" flag onto lines that already have "Block Everything" flag is that those lines will become bold white in Doom Builder.