Events will happen at your keep while you're away, but not on the scale of a siege, so there's no reason to just hide behind your stone walls and wait to be attacked. Getting out and building up your level like a normal adventurer will net you more prestige, which translates into more responsibility and more troops.
You'll also have a chance to turn the tables and assault enemy strongholds, although you won't usually have your troops with you. Instead, you'll bring along a party of three non-playable characters - each with distinct personalities, backstories and strengths - to order around. Since these guys are usually a lot tougher than your rank-and-file guards, they should be more than enough to get your back.
The four-player dynamic also extends to multiplayer games, more or less. NWN2 will support up to 64 gamers, but co-op in the central, story-based campaign is limited to four. The good news is that your non-player helpers can tag along, boosting your number to seven. And if you want a larger-scale adventure, you can always create one with the included construction tools.
When the game finally hits in October, it'll feature a huge story, a modular, customizable interface (admittedly inspired by World of WarCraft) and more depth than any D&D game to date. We still don't know how it plays, but after seeing it up close, we can't wait to find out.