If you're using mods then saving and leaderbords are disabled, due to the fact that Robot wants to, at the end of the day, promote fair play.

Hope this answers your question.

Ah, okay thank you for the answer ;)

i do not care about leaderboards, but why will they not let me save it in singleplayer? i mean this is more fun to me, and i thought this is the purpose of games, isn't it?

Agreed, I'm fine with them blocking leaderboard scores (I don't care about a score anyway), but I want to be able to play with my mods and save my game!

Very frustrating.

Torchlight 2 handled this issue fairly, and it would have been great if Orcs Must Die! 2 handled this issue in a similar fashion. In Torchlight 2, mod users save their game in a different folder than those that play without mods. That way, people can play with mods and still save their game. Yet, things like the leaderboards and achievements could still be locked for mod users.

The way Robot did it...well it defeats the purpose of having mods. Seriously.

I correct myself. Robot ALMOST defeated the purpose of having mods. The game will save your progress in Apprentice Mode (non-modded "classic" mode, that is). You can do things like buy yourself some new weapons and traps. Then you can load your save file from your non-modded game to enjoy the game the way you'd like to (i.e. with mods). FYI