In 0.16, a new and more difficult mega-ziggurat was added. After completing floor 27 of a ziggurat, the next ziggurat will be harder, packed with more monsters. After completing floor 27 of that second one, the next will be even harder. And so on until the 13th ziggurat completed, at which point the difficulty maxes out.
By maybe the 4th or 5th ziggurat, most levels will be entirely packed with a monster on every square.
The mega-zig requires a fairly particular character to succeed, although an alternative to this guide's template is discussed here: https://crawl.develz.org/tavern/viewtopic.php?f=12&t=17020
You should use Makhleb for healing on kills. You'll be taking huge amounts of damage and Makhleb is the only way to stay alive. As an example of how strong Makhleb is, if you are not in lichform, it's not uncommon to get tormented for half your hp and heal it all back the next turn with a well placed firestorm. Other gods such as Vehumet might be okay for the first few mega-zigs, but Mak is really the only good choice.
You have more freedom when choosing a race. Nagas are good because of their high HP and AC, and because their poison resistance and See Invisible give you more freedom when choosing equipment. Demonspawn can be good if they get good mutations, especially nightstalker, robust, or augmentation. Ghouls have torment immunity plus potions, but you'll probably have to use Jiyva to get more int before switching to Makhleb. You could try other races. I'd recommend against low HP races like Deep Elves. Deep Elves have high aptitudes, but that doesn't matter in the long run after all skills are maxed out.
Tornado, Fire Storm, Glaciate, and Necromutation are the essentials. Hit them with Tornado, and while you're waiting to recast it, hit them with Firestorm or Glaciate. Haste is a close runner-up. Darkness is very useful for cutting down on smiting/hellfire attacks. Sublimation of Blood is a great way to get more mp when you aren't forced to be in lichform. Disjunction can help keep you from getting hit too much by enemies with chaos attacks, to reduce the risk of paralysis/berserk. The rest are optional - you might want Borgnjor's Revivification, Deflect Missiles, Phase Shift, Stoneskin, Shatter, etc. Here's an example spell list:
a - Necromutation Trmt/Necr ########.. 1% 8 None b - Haste Chrm ######## 0% 6 None c - Swiftness Chrm/Air ######## 0% 2 None d - Controlled Blink Tloc N/A 1% 7 None e - Darkness Hex ########.. 1% 7 None f - Disjunction Tloc ########.. 1% 8 None g - Apportation Tloc ########.. 0% 1 None h - Tornado Air ########.. 1% 9 ##..... i - Fire Storm Conj/Fire ########.. 1% 9 ##..... j - Borgnjor's Revivific Necr ########.. 1% 8 None k - Sublimation of Blood Necr ########.. 0% 2 None o - Regeneration Chrm/Necr ########.. 0% 3 None p - Glaciate Conj/Ice ########.. 1% 9 ##.....
Wear a shield. Shield of the Gong is nice. Have as much AC as you can have, without hurting your spell success rates. 40 AC is enough. When possible, wear equipment that increases your int. You want to have 40-50 int. That's more than the minimum you need to cast your level 9 spells, but you'll spend a lot of time with your int drained as much as 10 points, so it helps to have extra. Also, armour that increases your MR is handy, since paralysis in a megazig is sometimes a death sentence.
A Staff of Energy and a Crystal ball of energy (CBOE), and plenty of evocations to use them, are the essentials. You can ditch the staff of energy and switch to the Staff of Wucad Mu after you find it. It's good to still use the CBOE when possible because it doesn't drain your int as much.
On levels without too many enemies that cast torment, you can drop out of lichform and use Sublimation of Blood, which has the advantage of not draining your int. Don't worry about losing HP, as you'll gain it all back with Makhleb. Don't try it on mummy levels.
Wear jewelry that increases your int. When you're using lichform, you need clarity at all times to avoid confusion when channeling with CBOE or Staff of Wucad Mu. You should also have sustain abilities for channeling and mummies.
Equipping for Silence
Silence is a very major threat and you need the tools to handle it. You need three essentials: Disc of Storms, javelins of penetration, and evocable blink. Disc of Storms is your first line of attack against enemies that lack rElec and can keep you alive through Makhleb healing. Javelins of penetration (if you have plenty of Throwing skill) allow you to take out silent spectres that are behind hordes of enemies. Evocable blink will help you quickly move out from your position and find silent spectres to kill, as well as letting you move at all while surrounded.
Staff of Dispater can be used in a pinch if you have evocations but not enough throwing for javelins of penetration. It deals great ranged damage while silenced, but can't be used if enemies are too close and can't hit silent spectres behind other enemies. Be very cautious around hellions if you have no other silent ranged option.
A broad axe is not really required, since it doesn't do nearly as much damage as disc of storms. But it can be nice to have if you really must kill an enemy that has rElec and don't mind spending a lot of turns doing it. Antimagic is good, distortion is good, and chopping is good (for those times when you don't want to deal with the consequences of antimagic/distortion). Holy wrath is not so good because it can't be used in lichform.
You'll get more scrolls of fog than you need, but Cloak of the Thief is nice to wear, because it lets you fog while you're silenced. It's for an emergency situation, like your lichform running out while silenced with enemies that cast torment in los.
Always cast Necromutation before descending stairs. If the next floor is Crypt-themed and you're silenced on entry with tormentors/curse toes/curse skulls/shadow fiends nearby, you'll be glad you did.
It's also helpful to cast Darkness before descending stairs.
If necessary, make sure to uncurse your essential weapons before descending, because you can't uncurse them while silenced. This would include at least staff of energy/wucad mu, staff of dispater, and your melee weapon.
Make sure you have enough charges on your wands, especially wand of hasting, because you can't recharge them while silenced.
Tactics in Silent Levels
For the first couple mega-zigs, silent levels might not be too dangerous. You might not be silenced on entry. Enemies might be asleep, allowing you to pick them off at your leisure with staff of dispater. However, stay in lichform at all times, because there are enemies can cast torment.
Eventually, though, you're going to run into a crypt level where you are silenced on entry and the screen is packed with enemies. Your priority here should be to kill enough silent spectres to get a patch where silence doesn't reach, so that you can recast lichform (and also blast away with spells). The thing to fear is your lichform running out while you are still silenced, which will result in getting badly tormented. If you have killed all silent spectres in LOS and you are still silenced, you need to go on the attack, either using evocable blink or just walking forward, until you find more spectres to kill. Use javelins of penetration. After killing them you can retreat back to the area that's no longer silenced.
Staying hasted (with a wand of hasting) is a good idea because it helps you accomplish more before lichform expires.
Only hit things with an axe if disc of storms is not working on them and they really need to die (e.g. curse toes). You can waste a lot of time hitting things with axes while your lichform runs out.
Tactics in Chaos Levels
A chaos level can be identified by the presence of many pandemonium lords. It will also have apocalypse crabs and killer klowns. The priority here is to avoid getting hit by chaos attacks, and avoid standing in clouds of chaos, because these clouds can paralyze or berserk you, leaving you temporarily helpless. So, use Disjunction liberally to keep enemies away. Disjunction both prevents getting hit by killer klowns and gives you a free space to walk into in case your square gets hit with a cloud of chaos. Check every turn to make sure you aren't standing in a cloud of chaos. Glaciate will make your own clouds, giving you safe spaces to stand.
It might be nice to wear stasis in a chaos level. However, you'll probably have to drop out of lichform to do so, unless you have an alternate source of clarity. Beware that some of the panlords cast torment.
Sometimes a panlord will cast silence, but this isn't anywhere near as bad as crypt silence and you can probably take him out.
Tactics in Pan Levels
A pandemonium level can be identified by the presence of demonspawn, with plenty of fiends and other pandemonium denizens. The main danger here is that demonspawn warmongers will sap your magic, leaving your attack capacity greatly diminished. To prevent this, identify and kill warmongers ASAP. Also note that Sap Magic requires an open line of fire, so you're temporarily ok as long as other enemies are in the way.
If you do get sapped, you'll only be able to cast a few spells before the -Wiz becomes excessive. Save it for the most important stuff, like recasting tornado and necromutation, and maybe darkness. You can use disc of storms in between tornado/necromut. Your goal is to survive until the Sap wears off. Fog can help, and so can retreating to a safer area, if possible.
If all else fails, you can drop out of lichform and quaff cancellation, which will remove the Sap and -Wiz. However, given the number of enemies that cast torment in Pan levels, this might be a mistake. Also, you get hardly any potions of cancellation in zigs so this tactic can't be used too many times.
Turn invisible, or just spam disc of storms.
A holy zig level is full of smiting, and darkness doesn't work due to the halos. Just power through it while hasted (and not in lichform). It's worth a mention because the sheer amount of smiting can actually compete with Makhleb's healing. Fog might be necessary if you make it to a high megazig.
Annoying! As long as you stay in lichform, they aren't going to kill you directly, but you are going to lose a lot of stats. Make sure you have sustain abilities. After you've cleared a bunch of them, you might want to just teleport to the other end of the level and exit, because the more of them you kill the more statdrain you will suffer. Potions of restore abilities are actually in somewhat short supply in zigs. Also, stay hasted because they will slow you.
Fire levels packed with orbs of fire can sometimes be a challenge if you don't yet have rF+++. If you're depending on the elemental staff for your fire resistance, you may have to stay out of lichform so you can use sublimation of blood instead of staff of energy/CBOE, and just deal with whatever mutations you get. Even if you have 300 hp, you can die pretty fast if you lack fire resistance.
Wear a ring of teleportation and constantly teleport around in Pandemonium to cover more ground, and if you find a portal to another Pandemonium level, immediately take it (because you cover more ground that way). Or, use magic mapping if you have any. You can recognize zig portals on the map because they're always surrounded by a rectangular or diamond-shaped border.