The possibilities in this game are still unmatched by modern games, from the conversation system, the day/night cycle of the NPCs, to the story. The spell system had less spells than many other games, but they were more useful and nastier (like invisible demons casting possession) as is often the case. If you have a copy of Dungeon Siege 1, btw, you can play a remake of Ultima 5 on that engine. The creators of that mod invested a huge amount of work into it (including a new orchestral soundtrack) and pretty much nailed it given the limitations of the engine.
If only someone would implement a more modern interface, maybe like in U7, I would still be playing it. I have many memories about what you could do - making gunpowder and improvising primitive guns is stuck there. Not very likely, but if someone does a remake, I'd go after Darden once more :)
