Note - I haven't played Havoc32 much. The layout seems similar, but I'm sure are some differences which change the strategies outlined below. Watch out for this.

The Goal - Your team gains points for damaging the enemy control room, located at the top of the base. Hit a button to open up the room, and start shooting! Any damage incurred will keep the room from closing, so fire often.

Choke Points

1. The Bridge - In the original Havoc, shadows easily hide the location of Snipers. The best way to frag 'em: use rockets (and other teammates) as a distraction. Once you're safely on their side, it should be easy to kill them, and any other Snipers which come along.
2. The Lobby - A Demoman's dream come true. In many games, the Lobby may very well be filled with lava. It is always filled with one type of grenade or another ... MIRVs, nails, pipebombs, etc. Surviving against a team with several Demomen and Soldiers is a tough task. This is the main, and probably only, choke point within the base.

Class Issues

1. Scout - <snicker> Yeah, right. Good luck surviving.
2. Sniper - The wings are a nice place to hide. Lots of nice shadows. However, once the enemy penetrates the area, you can pretty much say goodbye to your life. Another good place to hide: above the wings, in the shadows. Watch out for Soldiers and other Snipers, as usual.
3. Soldier - The Sniper-Hunter. Otherwise, beware inside the bases. REALLY narrow hallways and small rooms = lots of potential suicides. Probably best if you defended from the top level of the base.
4. Demolition Man - I have a feeling that Havoc was created especially for the Demoman. The only thing it needs is a dispenser which hands out unlimited MIRVs and detpacks. (It'd be interesting to see how this map was played with the Demoman removed) Anyway, turn the lobby into a virtual lava-field with grenades, pipebombs, MIRVs, detpacks, whatever.
5. Medic - I'm tempted to say "See Scout". But you'll probably survive somewhat longer. However, without any blast weapon (with the exception of the normal grenade), you're not going to be very important in this level. Healing is rather unnecessary, since it's not that far to go from your respawn point to the enemy base.
6. Heavy Weapons Guy - You'll be happy that you can withstand a lot of damage. If you happen to make it through to the top of the enemy base, you'll be able to saw most of the defenders in half and acquire quite a few points for the team. The hard part is, of course, getting there.
7. Pyro - Probably some increased frag potential in this level. Players are a lot more likely to be heavily wounded once they see some action (since it almost always has to do with a rocket or grenade). The problem is that once they've got a flame attached to them, players are still likely to be killed via some other (quicker) method.
8. Spy - N/A
9. Engineer - N/A

Things to Remember
To score, two things must be done. Open up the room. Attack. It's best to separate those tasks. Once open, keep dishing out damage and have teammates continually come in to replace those who get killed. Hopefully you will have racked up quite a bit of points.

Demomen, watch out with those detpacks! It seems like most Havoc games turn into detpack wars, much to the chagrin of pretty much everybody. Crossing the bridge can become damn near impossible. If the enemy pack doesn't blow you into smithereens, your teammate's will throw you into the lava. Ugh.