The following is a general guide that can teach players some simple strategies. This has been compiled based off of other guides and personal information.
You may think at first that ladders are simply transportation, but they can make the otherwise simple combat highly strategic. No entity (including you) is able to climb up a ladder if another entity is above it or at the top. Additionally, dropping from a ladder onto an enemy will stun them and knock them off any ladder they're on. Using these two tactics, it is possible to defeat any enemy even with simple weapons. If the enemy is below a platform you're on and there is only one ladder to it, it is impossible for them to reach you while you are on the space above the ladder. You can use this to your advantage by climbing down the ladder, quickly shooting or throwing a weapon, and then climbing back up. If the enemy tries to climb the ladder, you can knock them off of it. In open rooms with many ladders and platforms, combat can be very tricky as the monsters will try to employ some of these tactics as well.
It can be fairly difficult to regain health at some stages in this game, especially when in a fight. Hearts, sleep, and heal runes are the only easy ways to get health back. In combat, if you do not pay attention to your HP, you can be killed even if you have healing items in your inventory. The key is to open your inventory (pausing the game) frequently. This can give you a chance to take a breath and plan your next moves. You should also take advantage of hearts, runes, and other items that may save you. With a rune of undead, however, it is possible to survive a killing blow in a fight. If you eat it, there is a chance of you being resurrected as a skeleton. Skeletons like the minion are weak, so equipping the face of a beheaded monster can be used to raise your maximum health. While doing this, however, you will be unable to wear other armor. If possible, get polymorphed by RNG or a rune, or return to the Overworld to become a rogue (found through a portal or by going upstairs from level one).
Wearing a face will turn you into the monster that it belongs to. This is a mechanic that is useful later on, and especially for the minion. Some monsters have extremely good abilities that are specialized for certain play styles. For instance, goblins are quick and good for exploring, golems, trolls, and gnolls are good at tanking damage, and vampires can be used to regenerate health. Some monsters also have abilities that aren't apparent, such as drows having infinite vision. If you have a very good piece of armor, being polymorphed luckily into a good race is a way to get both benefits.
For any slightly long amount of playing, storing items in the Overworld is essential. It is the most easily reached of all worlds, and items left in it carry over into subsequent games. If you have a moderately good game, you should definitely consider returning to the Overworld to store your best items. Later these will give you a huge starting advantage. Runes will also remain identified if they are left in the Overworld, which saves much of the hassle of identifying new ones.
In the Overworld, there is a convenient healstone that can be used to rapidly regenerate health. There is also a grindstone that can be used to passively earn experience in early levels, although saving your quickenings can be even more useful in the dungeon to kill monsters and heal yourself. Something that might not be immediately obvious, however, is that the Underworld and cleared pocket dimensions can be slept in without fear, which will regain health with just as much ease.
Since everything in the Overworld is consistent, the most recently created portal to the dungeons will remain. This can be used as a jumpstart to the game. The linked portals also stay, making the levels familiar and good hubs for storage, transportation, and healing.
(This section may contain spoilers.)
Chaos is the lowest level of the dungeons accessible by the rogue. True to its name, the levels shift and serve to hinder transit and disorient the player. This will also lead to an excess of golems, which are very tough monsters. You can find high-level equipment, but equally powerful monsters to fight. You will need to be comfortable with using runes and having methods of identifying them, because you can use them to exponentially increase the ability of a piece of equipment. Their direct effects can also be used this way.
The Balrog is a very tough creature that serves as the boss of the game and holds the Amulet of Yendor. He will take a position by the stairs and escape if he wishes. You and your minion will have to be well-prepared to deal with him, which includes being well-stocked with hearts and runes to use. Some tactics used for fighting him can be found here.