I'm fairly inexperienced in mobile gaming, but I'll venture a few answers anyway:
When you run a mobile game on your device, do you prefer it to start on portrait orientation like you were using your phone in the standard way?
I think that I prefer the game to run in whatever orientation works best for the game; if the game works equally well in either orientation, then I think that I'd prefer that it respect whatever orientation I have it in at the time, switching when I re-orient the 'phone.
As to forcing the orientation, I have no problem with that at all, as long as the forced orientation benefits the game.
Now, I ask the same question again but with an extra info: the game is based on Eye of the Beholder series. You have the dungeon view and the little characters faces.
Hmm... I think that the answer, for me, at least, still depends somewhat on how you intend to lay out the screen, and how you intend to control the game. (You've said that your game is based on Eye of the Beholder, but that doesn't necessarily imply that you're using exactly the same layout as was used there. If you are planning on using that same layout, it might be worth reworking it a little to make slightly better use of the available screen-space.) If the dungeon-view is small enough, I could see a portrait orientation working. On the other hand, if you have a full-screen or near-full-screen dungeon view then portrait orientation might not work as well as landscape.