Now that we've got some of the concept art out of the way, let's talk about level breakdown, scene by scene.

THE SEDGEWICK HOTEL

You're trapped on the third floor, and Slimer's in the ballroom. Meanwhile, ghosts–roamers, tablecloths, and skeletal maids–are attacking the tram, so you'll need the Proton Cannon to zap them away. The perspective changes every so often, from an over-the-shoulder view to looking directly ahead of you as track and tram position to give you better firing-lines. Nothing too difficult here, as this is your introduction to the gameplay.

SLIMER

Again, nothing too dangerous. He's mostly an annoyance, flitting about and eating. The trick is to look at what he's going to eat next and blow it up just as he's about to go for it, but first you must lay your trap down. A cutscene establishes this by having your avatar destroy a wedding cake, sending Slimer reeling. Whittle him down to 0 and capture him!

THE CREEPY FUNHOUSE

What's this? You've stepped into something out of The Cabinet of Dr. Caligari, or even Killer Klowns from Outer Space, except it's all brightly lit and stuff…yeesh! Where are the Egg Pawns when you need them? Oh well, you'll just have to fight your way out. You'll soon learn that you can use the amusement park's rides and games to KO ghosts and cause splash damage. You'll need this new skill against…

KILLER JACK

It all started with a badly-timed Heath Ledger joke…now Killer Jack has emerged from the sideshow and is attacking you with his squeaky-hammer. Be alarmed, ‘cause he packs quite a punch. A punch? That’s it! Shoot the hammer-games to send the weight up to the bell, knocking him silly. When he's dazed, you can hold him in your Proton Beam. Careful, or else he'll wise up and get out of the beam…

Oh ok I know who you are talking about,he wasn't actually part of the Containment Unit battle,when the ride was redone,they added Louis Tully and a preshow,they also added in a Containment Unit and Walter Peck,the Hobo Ghost was part of Louis' attempt at scaring away Peck.

Speaking of which:

RUNAWAY GHOST TRAIN

As the title implies, you are hurtling alongside a runaway ghost train, towed along by the fragile power-cord of a Ghost Trap lodged between the railings atop the car. There aren't too many ghosts here, but those who are there will attempt to sabotage the Tram by switching its tracks, which will lead the Tram to plummet into the abyss below. Watch out, because the HOBO will try to attack you on his flying barrel (or whatever the hell he's riding on). His attacks will cause the Tram to fly off the rail for a moment, slowing you down and possibly allowing a ghost to switch the tracks on you.
If he tries to dive-bomb you, shoot the Proton Cannon and hang on for as long as you can before it overheats. When you've bagged this spook, the train will disappear, causing the Trap to disengage and retract back into the Tram.

THE HAUNTED DUNGEON

This level explores Castle Kildarby, filled with the ghosts of a terrible, sad history. You must work your way through the castle in the Tram as suits of armor and old weapons come to life and attack you. Fortunately, you will find lots of powerups (shields, armor-repair, etc.) to keep the Tram in good health, at least until you can reach the Dungeon.

BOSS: THE THREE-HEADED DRAGON

Since the Bird of Kildarby is no more, you must battle the beast of Kildarby, the Dragon. This ghostly abomination has three heads, two of which are much smaller than the main head, and can breathe fire and ice at you. The two smaller heads are its weak points, though aiming at them and getting a hit in will be no mean feat. Once more, the rules are the same: stun the ghost, then use the Proton Beam to whittle its health down until the counter reaches ZERO and you can trap it.

EDIT: This level would use a remixed and rearranged version of the SEGA game's Castle Theme.

THE TEMPLE OF GOZER

You made it. This is the end of the game. Now the big battle awaits.

GOZER

Gozer's health is set at 200, and she will use lightning to hurt you and impressive acrobatics to draw your fire. If you can somehow manage to get her down to 150, she will disappear. Looking into your mind, she will latch upon the first thing you think of and use that to destroy you…

THE STAY-PUFT MARSHMALLOW MAN

Mr. Stay-Puft has five health-bars. Significant damage to him will cause him to lose one health-bar and disappear. Gozer will reappear and you must do more damage to her. Rinse and repeat until either Gozer or Stay-Puft is defeated. (NOTE: It doesn't actually matter if you defeat Stay-Puft entirely, but if you do, you will earn the Red Stamp as a bonus.)

UPDATE, 8/5/09:

I've finished a level-design sketch (more like a walkthrough, really) of the Sedgewick Hotel level. I'll scan it sometime tonight, maybe.

My internet connection is working better than normal tonight, so here's the first, very basic, level design walkthrough for the Sedgewick Hotel. I haven't added the camera angles in yet…

And the Runaway Ghost Train walkthrough concept. This is designed so that you're looking down with a bird's eye view of the track and how it winds around. I should probably redesign it with a few more twists and turns, but this is the general idea.

The Haunted Castle level. Not quite to scale, and some areas are “invisible”, but you get the general idea.