Finally, Till is correct! The game is called "Siege of Avalon", but it has nothing to do with Celts, Saxons or Vikings. It is set in a fantasy world where a nomadic half-orc half-human race is invading the civilized world. The seven human kingdoms unite to fight them together, and the protagonist goes to their most important fortress, Avalon, which has been under siege for several years. There he helps the humans to win and fulfills many quests.
The game plays similarly to Diablo II, but wasn't received too well by fans and critics. I played the first two of the six chapters of the game, and although it was quite fun, what annoyed me most were certain bugs and inconveniences. You had to do a lot of running about, and sometimes it wasn't clear what you had to do next. At the end of chapter two I was meant to meet someone downstairs to proceed to chapter three, but I never found him, and had no idea how to continue, so I stop playing.
So now it's either Till's turn or steviejay's, since he got the Avalon bit right and Till already asked the previous question.

You two just settle it between yourselves.