Cursed Lands

I didn't level up magic... I only leveled up military skills, and I went against the guards with a sword and spear (I took it from someone in a cave)...
I don't remember the difficulty.... since I didn't change it.. what is the default?

If you are playing through the game for the first time, only hard difficulty is available. As for what comes next by default, I don't remember. But even when leveling up spear and melee combat, considering agility, your attack ends up being no more than 121. And an Imperial on hard has defense, if I recall correctly, of 140. It's possible to kill them, but with very long save-scumming. However, with acid fog, it will be easy and without reloads.

Maybe that's why my warrior was weak, because I played on hard?

2Ataman
Even on hard, spiders are easily taken down by a warrior in a fair fight; most likely you missed some leveling somewhere.