Klaus, I disagree.. and there are probably a number of threads about this:
Option 1:
- Do not 'start the encounter' until the sentry notices the group... roll for surprise and initiative. No readied actions can occur but the PCs all act in the surprise round.
Option 2:
- start the encounter when the group is aware of the NPCs, allows for readying actions but there is no surprise round. Run the PCs in initiative order until the Sentry notices them, in which case the alerted NPCs join the fight and roll initiative. This gives the PCs most of a round to act before the sleeping characters are alerted.
Other issue, which there are also a number of threads on, sleeping. There are no good rules for waking up from sleep. My HR is that waking up is a Move action, which means ambushing sleeping characters often grants the ambushers almost 2 rounds of actions before the poor targets can effectively react {unless they fight from prone and have all thier gear ready while sleeping}
Last issue.. is the targetting beings outside of the light of a campfire. Thats probably the DM forgetting environmental effects of night
Done that a couple times myself... otherwise both Archer and Rogue would have complete concealment and could not be targeted.
Which, of course, would make for dead bad guys pretty quick. Just remember... it also makes for dead PC's when the situation is reversed