I don't see how you guys can think this is a high level campaign. Basic enemies off the top of my head are Imps, goblins, humans, skeletons, gnolls, & spiders. None of those scream level 5 or higher...ever
You forgot phase spiders, the matriarch, hag, redcaps, bulette, minotaurs could all be considered level 5 and up. The only reason they can be destroyed fairly easily is due to Larian combat.
In a straight up 5e ruleset fight, some of those encounters would be very challenging at levels 1-4.
Those are in small numbers so a single or couple in one encounter would be considered low, while a large number of weaker creatures would equal that same encounter. (I couldn't remember the name for redcaps though, thanks) So again the campaign does not seem high end at the moment.
On a side note, you can hit level 4 before even doing the swamp, underdark, or whatever is passed the gith on the road, I actually haven't done any of those areas at all yet.
I get your point and in all fairness, you are right. Goblins, gnolls and such are low level encounters. And the bulette, minotaurs are meant for 5th level characters which you should reach by the time you see them. I think it's more about timing and execution. You start off in the middle of a fight between mind flayers and gith. I think players should be eased into stuff like that instead of dropping it first. Let players get comfortable with the game with basic enemies like you noted then go nuts.
FYI, you don't just face one or two redcaps. Since you haven't encountered them yet, I won't go into details. Same with the phase spiders.
Swamp is a side story area that can be done at level 3-4, which from what little I've done / experienced, it's probably set for around that level (my multi-group did go to that area but it was rushed and never finished, we got a bug related to the fog causing ctd every time loading a save from that area) But that's just me guessing. Redcaps are equal to goblins as in they are a minor creature that can be spammed for level 3-4s, again I'm presuming (I'd have to get my books out and look but I only have 3.5 and 4th editions).
The phase spiders come in 2-3 groups, it has been a bit but if I recall you have the boss spider and two phase spiders in the main encounter plus additional groups (2-3) in the cave.
The intro of the game you only face imps, humans, and whatever the other creature is. The other two at the last encounter are setup to put a time limit at the end, make it feel short on time.
Now that I'm thinking about the whole battle at the ship, the items on the tables probably should be scattered on the floors instead of being on the table like they are.
