I'm wondering how much leeway I'll still have for side quests by the time I get back, and whether I've actually gone a tad too early with my group... though some of the fights are trivially easy, we're finding others to be insanely difficult...

SPOILER!
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You can still do the side quest after you freed Imoen..
And the best level to go there, would be at one were you don't get killed [img]tongue.gif[/img]
No seriously, it depends on who's in your party, and what class you are..
But everything after level 14 should work [img]smile.gif[/img]

If you can beat the Vampire Lair, you good enough to take on spellhold. there are a few quests in the town as well, but not enough to level someone up unless their most of the way there. or maybe if their playing solo.

Also, keep in mind Spell Hold isn't the only problem. There are still many fights before you get back on home turf.

SP?
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The UD is quite tough with the beholder lair and the Mind Flayers place up there for the toughest fights in the game if unprepared. I also don't recall any Temples down there, and I recall carrying the belongins of my fallen brethren for quite some time since I had no resurrectors or applicable items in my group. It is always better to go over there stronger, and finish up a couple quests on main land.