welcome to board rawhide!
[img]graemlins/happydance.gif[/img] rollin rollin rollin...
it goes without saying, response contains spoilers...
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regarding the cave-in, i've never had any trouble following the path through the wall. try getting a running start - perhaps you are getting stuck on a raised bit (sometimes the 3D nature of wiz 8 can get a bit narky in this respect).
ghoul's tip on the dungeons is your best course. suffice to say you do have to have all the runes activated at once before you can take the next step towards entering them.
finally, the priest in wiz 8 is akin to the classic PNP character. they are a great utility character to have along. they key for them is buffing and healing - it's virtually a full time job keeping a party alive! and although their offensive magic is generally either weak or cant be cast frequently due to low mana, there are a number of people who claim the priest is the most valuable class to have.
atlus 7 did some great work outling two ways to develop a battle priest that focusses on fighting stats/abilities (eg mace and shield or 2H staff) and lets magic develop come naturally from spell casting.
the other options is to run up powercast with the intention of maximising the effectiveness of those late attacking spells the priest gets (eg lightning, falling stars, death wish). they're costly spells, but with a priest with high power cast throwing a death wish (divine) along side an alchemist with high power cast throwing quicksand (earth) and a mage with high power cast throwing asphyxiation (air)... whew. that'll put some hurtin on any bunch. however, that comes at the expense of the priest's fighting development.
if i had to pick a weak spell book, i'd say psionic. through level 4/5 is great, but the level 6/7 spells are costly and minimally effective. i'd take a monk over a psionicist every time.