Ancient and medieval fortifications are often shown to have wooden gate doors in the midst of their stone walls. Now granted, these doors are often massive and incredibly thick, but it still seems like a very risky point of weakness for an otherwise imposing structure. A splash of oil and a flaming arrow seems like it could leave your castle with a giant gaping hole in the wall in a matter of hours.
And yet, that didn't seem to happen. Or at least not regularly enough to require a different method of constructing doorways through the ages. Attacking armies often had to bash down the gates with rams, or scale the walls rather than going through the gates at all, to get inside a city, castle, or fort.
Why was that? What methods were used to protect the wooden elements of fortifications against fire? I've heard of siege towers being covered by animal skins for that purpose, but never fortification doors. And while thick doors like that would take a long time to burn through, it still seems like it would be vastly easier to splash some oil and throw a torch than to undertake more "conventional" siege methods.
Unless, as it seems, that wouldn't have worked?