Yeah, get to level 5 with most anyone, you can bruise through at level 4 with the Priest though (might want to anyways, she takes ages to level up).
Specifically speaking...
In the first dungeon, go down the first rope and you will see some Orcs up on a small platform, with some Green Slimes that drop from the ceiling and moving platforms to the far left. Stay on that raised platform and walk back and forth, slaying the Orcs and Green Slimes. From there you can proceed across the moving platforms (look out for bats) and into the next room.
In the second dungeon, go down the first rope (careful not to go into the water, it'll hurt you at first) and jump off to the left. Giant Frogs will come after you, just kill them and don't waste time with them really. After them comes the Trolls who throw bones at you. They can be hard to deal with on the decline, but get close enough to them so that their bones won't hit you, then attack. You should be able to wipe them out without getting hit. The first part just has them coming at you on one plane so they're easy to take out but they're few. The second area has them coming in groups but the area is sloped. At the bottom of the second area is a Treant, just kill it as soon as it pops up. Later in the level, where you find the Blue Door, the platform drops down a bit and you'll find three Treants, this is also a good place to do some levelling. Once you drop off of this platform, you won't be able to level anymore (unless you're the Priest) until after the next boss.
The third dungeon has the best level-up area in the game. When you enter, you go all the way to the bottom, then go all the way left. You'll run into a large set of stair-like platforms with Rock Golems walking slowly towards you. If you turn your back on them, they'll double their speed in an effort to run you over. They take quite a beating at first but stick with it. I've used this area to get all the way up to experience level 20 (not really necessary here but it's fun going through the rest of the game at max level). Be sure to go back to the Inn in the second overworld if you're running too low on life though...try not to go back while too low because of the nasty Beholder-kin you have to fight along the way.
The fourth dungeon has a couple of good levelling spots. The first one is in the first section after you get to the bottom of the rope. Manticores will come at you from the right, they have to be hit while crouching for most characters. They give pretty decent experience. Way to the right and up the rope (past the crazy guy who runs past you) are some dragons. They give some of the highest XP awards in the game but aren't particularly easy to fight at first, especially if your defence is low (you get hit, you go flying hardcore). But once you get the method down, they're good killin' for points. Don't bother with the Zombies or Flares, they're worth squat and they're too much of a pain to kill.
The fifth dungeon lacks any real good levelling spots, except for the tower climb after the Worm has been killed...stronger Treants appear on unreachable platforms above you, and are sitting ducks if you play any character but the Fighter (he can fight them too but it's not as easy).
Of particular note here in comparison to the arcade version...the Treants in the PCE/TG version will stop animating when being hit, meaning you can often times kill them before they shoot at you. In the arcade original, this doesn't happen, and they will blow you away in short order if you don't have the right timing to dodge their shots (not as easy as it sounds). Use this to your advantage when levelling up in the PCE/TG version.