Level 15. By the time that they're finished there, they should be level 17 (according to the next adventure).
Well, ST is written for 3.5, so depends on your conversion (if any) to PF.
Generally people have converted it so that it stays in line with the 3.5 ST, so it's level 15 when the PC's first visit Scuttlecove.
My group will be 15 when they head there (they're 12 now)
One rule of thumb is that a L 15 character in 3.5 is at the same power level as a L 14 character in PF.
If you're running a 3.5 AP with the PF system, you might want to beef up the NPCs, etc. For example, in 3.5 an NPC Rogue 15 was considered to be CR 15, while an NPC Expert 15 was treated as CR 14. In PF, they are CR 14 and CR 13 respectively.
Another thing to remember is that the 3.5 APs - both the three in Dungeon (SCAP, AoW, and STAP) and the first four Paizo ones (the original RotR, CotCT, SD, and LoF) use the 3.5 XP track. The closest equivalent in PF is the Fast XP track. (Although the Anniversary Edition of RotR was converted to PF, it still uses the Fast XP track - unlike all other PF APs.)
One rule of thumb is that a L 15 character in 3.5 is at the same power level as a L 14 character in PF.
I don't think I agree with you here Bell, though it varies by group. A heavy or moderate optimization 3.5 party still generally smokes a PF party. There were lots of toys available that are simply impossible to reproduce now (Incantatarix, Arcane Thesis, Assay Spell Resistance, Orb Spells, Power Attack shenanigans, chargers, frenzied berzerkers, celerity, dragonsight, anticipate teleport, and so forth). That's before we get into how many spells and effects have changed from 3.5 to PF, generally previously granting immunity or instant fixes (protection from evil, remove poison, ect) that now possess opportunity for failure and much more limited utility.
Now if you were a limited source group that smacked down people who dipped into multiple prestige classes and kept people from taking advantage of the strongest options things might be different, but most groups I saw encouraged or at least allowed people to take advantage of the options WotC kept giving people.
If it's run as written, the city should not be too tough for anything over 12th level.
If it's restatted to be appropriate for its CR, it will provide a challenge to 15-17th level parties.