Reaching Havel Tower is fairly simple. From Forest City, travel east one screen, and then keep going north until you encounter the tower. In preparation for the tower, you should be level 5 or above. At this level, you are hopefully strong enough to add Leather Mail and a Shield to go along with your Helmet and increase your Armor Class. While this won't make you invulnerable to the monsters in the tower, it will dramatically cut down how much damage they can do to you.
There's only one other item that may help you, and that's a sword. The only place to obtain one that you can carry is in the Subterranean City, which you need a fairly high Intelligence stat to access. A sword is useful, not only because it's strong than the club, but also because it has longer reach. So long, in fact, that most versions of the game allow you to attack monsters through the walls where you are relatively safe from counterattack. This gives you a tremendous advantage in battle and makes racking up experience points much easier.
Before you enter, it's incredibly helpful to have learned the Move spell, as that can instantly whisk you back to an Inn, and safe you the labor of exiting all the way back out of the tower, and across the land until you make it back to a city.
The first floor of the tower is divided into 9 screens arranged 3×3. You arrive in the middle of the bottom row. You will encounter three of the tower's inhabitants on this floor and the next. While the Minor Slimes outside are evil, the regular Slimes in the tower are not, so avoid attacking them. The same is true for the Vampire Bats which fly harmlessly around the floor. The only threats are the Balu Balu creatures. These net you 15 experience points per kill, but they are capable of shooting you. They shoot projectiles which travel straight, and can do a good amount of damage if you're not well protected.
To reach the staircase to the next floor, you'll have to make your way to the upper left corner of the floor. Doing so will not be easy as there is no straight route to get there. The narrow corridors twist around, taking you through every screen before you reach your destination. To make things worse, slow moving enemies like Slimes sometimes block your progress and you're forced to wait for them to move out of the way. However, there are a number of walls which have hidden passages in that that allow you to shortcut through the floor. In Super Hydlide, you can find one such shortcut on this floor in the center screen, near the top of the wall which runs up and down through the middle of the screen. This allows you to cut out a good portion of the journey along the right side of the floor, saving you precious time.
The second floor is occupied by the same monsters as the first floor, and is just as labyrinthine. You begin in the upper left corner, and have to make your way around to the lower left corner and then up to find the next staircase. Along the way, you'll have an opportunity to detour into the upper right corner of the floor to locate a treasure chest and obtain the Talisman. This item doubles the duration of spells that you cast. This is great for the six advanced spells that the Cleric and Monk can learn, but useless for the Fighter and the Thief, so those two classes should consider selling the Talisman for a good sum of money.
In the very middle of the floor, you'll find an elevator door which won't open. You'll need to find a way to activate the elevator in order to use this time saver. As with the first floor, there is a shortcut in Super Hydlide that you can take advantage of. On the screen below the elevator, you can find a secret passage through the wall which separates the upper right portion of the room from the remainder. Near the bottom, you can push left to walk through the wall and be that much closer to the stairs that lead to the third floor.
The floors begin to get tougher, and they narrow a bit. This floor trims down to 4 screens arranged in a 2×2 formation, but it is no less maze like. You begin in the lower left corner. The stairway to the 4th floor is also in the lower left screen, but you have to walk all the way up, right, down and around to reach it. Along the way, an elevator can be located in the lower right corner, but will remain inaccessible until you power it on.
The big thing to note about this floor are the enemies. Vampires and Elgars join the Balu Balus to make a floor composed entirely of evil monsters that you can defeat. However, these monsters are dangerous. Vampires walk around, and they can only attack at close range if you stand in front of them, but they can do a lot of damage with these attacks. Elgars look like a different color of Balu Balus, and they are far more aggressive. Their projectile isn't as strong as a Balu Balu's, but they will stop to fire at you whenever they get close enough.
There are no treasure chests found on the third floor, but if you happen to be walking around the far upper left corner of the floor, you might hear a noise whenever you step on a certain tile. That noise is an indication to take a close look at the spot. In many versions of the game, you will find some sort of "gift" or "pack". This item is one of many "joke items" thrown into the game that you can't actually use for any real purpose. Many just exist to bog down your inventory and should just be thrown away, although some can be sold for a good bit of money. This gift is one such item that is worth hanging on to for selling.
Floors 4, 5, and 6
Starting on this floor, the Balu Balus are replaced with Dark Eyes, which do considerably more damage, but are worth more experience points per kill. If you're playing one of the original Hydlide 3 games on computer, you'll want to make it up to the sixth floor and locate the switch for the elevator. However, this can be found on the 4th floor in the console versions of this game. In both versions, stepping back on the mechanism that starts the elevator causes you to fall through the floor to a level below.
Once you activate this switch, you can begin using the elevator to get higher up in the tower. The lowest floor you can access the elevator from is the second. In the mean time, use floor 4 to attack many monsters and level up your character.
Beyond Floor 6
Above the 6th floor, the floors all become single screen floors, and there are no monsters. All you typically find are two stairwells, one going up and one headed down, and the elevator in the middle. What you find near the top depends on what version you're playing.
If you are on the original computer versions of the game, you can visit floors 7. 31, 46, 100, 113, 176, 191, and 197. They are, for the most part, empty. Although you must search for a key item, the Cloud Stone on one of these floors. It can be found in the lower right corner of the 176th floor.
However, if you are playing any of the later computer releases, like the NEC PC-98, or either of the console versions, you will encounter a boss all the way up on floor 200. This boss is known as Hell Storm. This boss is rather simple. It comes on the screen from one direction. It looks for an opportunity to shoot at you, and then turn 90 degrees to exit back off the screen. Standing in front of it is asking to get hit. Rather, you should seek to lure it into firing, and then run alongside it or behind it and stab it as much as you can before you fall off the edge of the roof. Then reset yourself to the center of the roof and prepare for the next pass. Nail it as much as you can with each run until its health is depleted. A chest will appear in the middle of the screen. Open it to find the afore mentioned Cloud Stone.
Now you just need to figure out what to do with it.