When starting a new character you can either choose normal, nightmare and hell. But the two latter ones are impossible to play with a new character. How did they (developers) think this will work when you can't even kill the first enemy when entering the castle?

maybe they thought people would be smart enough to choose a difficulty where they can kill enemies ...

Typically nightmare should be blocked until level 20, hell until 30. At that point you will be more than capable of killing enemies. You may have an old un-patched version that doesn't prevent you from joining the inappropriate difficulties.

*Edit* as pointed out by advowson the OP is likely playing in hellfire.

You can start a new game with an existing character, and your existing character may be advanced enough to play readily in the higher difficulty levels.

According to Jarulf's Guide, in Hellfire, it is standard that you can always enter any difficulty level. In Diablo, it is standard that you were never able to enter higher difficulties with low level characters.As far as I know, the level restrictions were always enforced. This was not added in a patch.

Thanks, I never really played hellfire. My post indicating that it was something that was fixed by a patch was a guess as to how he was doing this. While I would like to play more mods, for the time being, I'm more focused on D1 Vanilla PvP and keeping the blog up to date.

The bonus difficulties were only available for multiplayer (if we're talking ONLY D1 and not hellfire). I could be wrong though. I think if you play in lan it fixes that?

This has to be some sort of trolling.

No it's no trolling you stupid!

LAN is a form of multiplayer. It lets you play with other people on the Local Area Network. All the multiplayer forms allow choosing the higher difficulty levels, if the character level requirement is met.

Which then makes it funnier.

Appearently it's possible but it's nothing that Blizzard had implemented in the original version without doing some rundabout thingys between single and multiplayer. It's very hard to kill the first monster but once you do it's possible to gain several levels.

In Diablo, but not in Hellfire, the higher difficulties are still pretty easy in Church. Monster HP is boosted in higher difficulties, but the formula depends heavily on the monster's base, so low HP Church enemies remain low HP in Hell difficulty. A mage with charged bolt and firebolt can probably kill a few at range, and a rogue might get in the needed lucky shots. A new warrior would be in big trouble trying to fight in melee though. Per above, even a few kills will deliver substantial experience.

In Hellfire, the difficulty adjustment for HP applies a much larger fixed bonus, so even Church monsters have significant HP in Hell difficulty.