What is this level? Is this level the respective chapters in BG?
Look, I'm not trying to be obscure.
You know how in a bioware game, when you raid a cave full of orcs, you proceed from room to room and the orcs attack you whenever you get too close, or whenever an orc makes an "alarm" noise and they're close enough to count as hearing it? Eventually you kill the orcs in every room until you find the room of the Boss Orc, and you kill him and take his treasure chest and rescue his captive or whatever the mission was supposed to be?
The "level" is the raid on the orc lair. Once its started, in a traditional bioware game there aren't that many major choices to be made. You basically just raid the lair. The biggest exploratory choices were made when you sought out whoever gave you the mission to raid the lair, hunted down the lair's location, and decided to complete the "raid the lair" objective. During the raid, the only big things you decide are how and when to draw aggro, whether to use consumable goods, and when to retreat and rest in order to return and continue where you left off as if nothing ever happened.
That's not the only way things can be done.
Replace the bioware style raid with you approaching the cave, fighting the orcs who guard the entrance, having some kind of intentionally designed scene inside the lair, and then some final showdown with the boss. Each as discrete scenes. No monsters are attacking you because you walked too close to where they were idly standing, awaiting a hero walking nearby.
Some of the big decisions from the old school bioware model would be changed.
"When and how do I draw aggro?" would be largely gone under this model. Because each fight is a self contained scene, you'd instead choose which fights to encounter (within limits of plot, skill, etc). So you might choose to sneak through one area, encountering a stealthy, assassination encounter, instead of charging headlong into a different area, encountering a big melee brawl. But in each fight, how and whether you draw aggro would be controled by the scenario, much like in regular, tabletop D&D, where you can't fight half the monsters in a battle by just staying 11 spaces away from the others at all times.
"Whether to use consumable goods" wouldn't be affected. It could be kept or left as the needs of the game and opinions of the designer decide.
"When to retreat and rest" would be a non issue. No one retreats to take a nap in the middle of raiding an orc lair! Retreat, if available, would constitute some degree of failure, and would be treated by the game as such. Either it would not be possible (you're too far in to back out without getting cut down as you run), or it would count as losing the scenario (a plot based reason why you could no longer win the scenario would explain this), or it would create some customized drawback such as reinforced guards or an inability to fully complete the objective.
If you really, really need examples of how this would work, consider regular D&D. You are either in or out of initiative. When you're in, you track things one way. When you're out, you track things another. You don't encounter monsters just standing around in their own homes waiting to be cut down (NVN was a big offender in this regard, you'd raid the homes of corrupt nobles to find large dining rooms set in architectural alcoves that hold nothing but 16 heavily armed mercenaries and a table). Leaving a dungeon or a castle or a series of encounters and then coming back results in things having changed while you're gone.
All of this is possible to program. From the perspective of a programmer, if you divide things into discrete scenes of combat or negotiation or investigation, coupled with travel and decision making in between, you really just have a flowchart.