Is a spellcaster the only way to win in this game? I stared as an archer, but on some video's it seems like you will have to have spell's (invisible etc) to actually make it through.

No, but you will want to get items that cast spells.

There are potions that cast many of the spells (including invisibility), as well as magic items that cast many others. Remember that there's no coded limit (as far as I can tell) on the number of any type of potion you can carry (though I would recommend not going over 30,000, as I don't trust the game to guard against integer overflow). Note that Potions of Restore Power are useless for non-spellcasters (though they are *incredibly* handy for Sorcerers), so you can ignore them (they restore SP).

It's also worth noting:

* Magical items can be repaired at weapon/armor shops to restore their charges.
* The contents of shops and Mage's Guilds do not change from playthrough to playthrough (though items you can't use will not appear in stores).

You can find lists of items sold in shops at This can be useful to find an item to cast the specific spell you need.

(I think there's one region for which some of the lists are inaccurate.)