I once setup what I thought was a formidable challenge for a lower level (3rd to 4th) party during a 2e campaign.
The party absolutely had to get to a small town several miles beyond this deep ravine/canyon. Fortunately there was a road with a stone bridge across said ravine/canyon (it ran for miles and miles in any one direction with a raging white water river at the bottom) at a narrow point. Unfortunately a group of bandits had moved in and fortified the bridge. The near side (from the party's perspective) had a swing gate and 5 bandits posted gaurd. The far side had an additional 10 gaurds with bows/crossbows and melee weapons standing in front of an earthen wall with a trench behind that (and a drawbridge to allow passage for people who paid the "toll") just beyond the end of the bridge proper. Furthermore there were two wooden watch towers (with additional archers) just beyond the trench (on either side of the road) with a barracks a little further back.
I figure the party would just pay the toll or get eaten up by missile fire if they tried fighting through. Instead the magic user in the group, casts Levitation, then Shield, rises up into the air and uses up a Wand of Fireballs to take out the gaurds, gaurd towers and barracks at the far end of the bridge, while the rest of the party engaged the bandits at the near end of the bridge.
The whole scenario showed me how effective several low level spells can be if used in various conjunctions, given a clever enough player. I've since found better ways to part pc's from thier coins.
