The early game is a great way to explore the envrionment and mechanics of the game. As there is no time pressure (beyond one found on an individual map), you are free to do whatever you wish. Furthermore, monsters are generally weak and any damage you receive can be receovered quickly.
At this stage, the main attribute you want to cultivate is Battle skill - the exact value is shown during the level generation screen. This is improved simply by killing monsters; netting you up to five points per kill. Collecting chests is also essential; they will give you the spells needed to survive later in the game.
Don't worry about traps - any traps that are lethal at this stage probably indicated a weak character, as does any monster that survives the player's attacks. In these cases, simply restart the game.
Temples serve limited purpose on the early levels; while they accelerate healing and levelling, hit point loss at this stage is normally recovered by normal dungeon exploration.
You can progress downward after clearing each level. If you see a monster arrive from a lower floor, that is your cue to go to the next level before these monsters become too dangerous.
Between level 5 and 10, you'll notice that you don't always maintain full health; this is perfectly normal as you should only need enough health to deal with critters that are nearby.
At this stage, feel free to start taking naps in temples if you get injured. While you can't get maximum HP, you should at least handle about 50, which is good enough for most encounters at that level.
At this stage, from level 10 onward, monsters hit hard and move more quickly. By the time you reach level 15, you may only get a few steps before monsters move around. As such, the Light spell becomes valuable for detecting enemies before they attack (potentially draining experience) without any significant loss.
With the large presense of gold, temples can now be used to fully heal the player - every 2000 gold sacrificed will provide this benefit, and you can obtain more gold simply by switching levels. However, this is time-consuming due to the constant monster movement.
Returning with the sword
Eventually, you will have to obtain the sword in the maze-like level. In some cases, you may find the sword is on floor 16 - that's lucky since you won't encounter demons or mages that steal things from you.
Your return trip will be difficult - if the humanoids attack you, they will steal the sword, and the only way to recover it is to return to that level. As such, light and invisibility spells are extremely valuable for avoiding any contact, and teleport spells can get you away from enemies that somehow slip through.
Rather than trying to complete the game, some players may prefer to dive as deep as possible. If you decide to do this route rather than trying to finish the game normally, you will encounter very powerful monsters that move quickly - you get exactly one step before they start moving, and any form of idling may cause you to miss your turn.
At this area, you'll have no reliance on spells - the only stat that matters is Battle Skill.
You will be extremely vulnerable until you find the temple - at that point, you can sacrifice your gold to become blessed and receive full healing.