Here, broadly, are some ruminations on the tools in the PC's toolbox, for dealing with Felltower. How did I build them into Felltower, and how valuable are they?
After all, if I don't account or allow for something, it's unlikely to work. And if I rank something as important and it's not used, the players are potentially missing on an expect aspect of the game and the benefits of it.
Some things in Felltower need to be fought. They're beatable, and you really need to beat them to get the things done that delvers want to do - explore and (more importantly) plunder.
You don't want to fight everything in Felltower, though. That's a sure route to a slow, miserable slog of sessions with lots of casualties, seemingly endless rest times, and coin flowing in rivers to the alchemists who crank out healing potions as fast as they are able to concoct them. You definitely need to pick and choose your battles, even if eventually you want most of the things you meet dead. Some picking and choosing is just going be "for now" - who do you want to or need to fight at them moment?
It's not a universal tool. There is at least one "thing" in Felltower that literally can never be beaten in the traditional sense. Frustrated, avoided, put out of action for a while, or dealt with, but not engaged in a straight-up fight and beaten. Plenty of "things" aren't worth the fight for the reward, or fighting them will cost potential value attainable by not fighting them.
This puts combat as a very useful tool - lots of stuff that needs beating and which can be beat. It's probably the most useful tool, for in-game and out-of-game reasons. In game, because a dead monster is less threatening than a live one. Out of game, because combat is fun when it's going well or turning out to be tight and eventful. The game is designed to ensure combat is an important tool. While Sun Tzu is right, the acme of skill is winning without fighting, part of the game fun are the fights that happen. Some of those fights will come with costs that must be paid. Part of the challenge is dealing with those when they occur, or when the dice tell you the price.
There are plenty of things to negotiate with - or just flat-out distract with bribes - in Felltower. Many "things" are eminently worth talking to and some are even so worth talking to you'll lose out if you avoid them or fight them.
But again, you can't negotiate with them all. Some things just won't negotiate. Some things can't even conceive of what negotiating entails. Some are so hate-filled or evil or just tricky that negotiating is a route to disaster no matter how well you think it's going.
This makes negotiation a useful tool. Lots of stuff is better spoken to than killed. But it's a secondary tool to combat, overall. Given the goals of delvers - loot and reward - it's hard to only negotiate to get them. Still, negotiations are fun and interesting, so they are built in to the game as a valid tool. It can even elevate to more important than combat if you negotiate with just the right parties.
There are some player-facing puzzles in Felltower. Trick doors, revolving statues, odd mazes, teleporter arrays, actual explicit "solve this riddle" or "solve this puzzle" type situations, and so on.
Unlike the previous tools, they always work in the situations where they work, and don't really work outside of them.
You can't treat everything as a puzzle. Some situations are set up as hard fights or tough negotiations, and aren't puzzles per se. Good tactical choices or clever negotiations might make them easier or even easy, but in general, they are what they are: hard fights, tough negotiations. The explicit puzzles, though, generally resist brute-force solutions well enough that they don't work. Or the brute-force solution takes vastly more resources than the puzzle solution would. Puzzles are rarely disguised, too - they're pretty up-front about being an odd thing to deal with and solve, even if the solution (or the reward) is far from clear.
Puzzles are there because they're a fairly common occurrence in the materials I drew on for the game. Puzzle solving skills are critical to confront them, but puzzle solving skills are of less value outside of them. This probably ranks puzzle-solving skills near the bottom of the value spectrum. It's rare for such puzzles to block your advance (although at least one does), but more common for them to shield special rewards, special areas, and special opportunities. Most of them are solvable primarily - or solely - through player reasoning or player trial and error. The puzzles are player-facing, and it's not a question of trying and trying again until you roll a 3 or flipping though GURPS Magic until you find the Solve This Puzzle spell.
Magic in General
Magic is a critical tool in the Felltower toolbox.
One problem with magic is that you need to rely on it, and it's very binary (works/doesn't work), yet it's not reliable. You can't depend on always having it. You can't even depend on it always working even when the spell succeeds. You can be led astray by false divinations, or tricked by your own assumptions about what success or failure means. It can distract you from a clear non-magical solution because there are so many choices of magical ones.
Plus, it's costly. It costs energy and eventually time. It's a nearly-universal tool but most problems solvable with magic are solvable without it, given different resources and time. Recovery from spells takes time, so using too much magic can slow you down and add more problems (wandering monsters, local monsters making adjustments, draining of resources) than it solves.
Magic is right up there with negotiation in terms of importance. You need it, you won't get far if you don't use it, and some problems aren't solvable as puzzles or with negotiation or combat. But it's a tool that acts like a hammer and nail situation - given sufficient magic, it's easy to fall into the trap of thinking there is a spell that solves every specific situation you encounter.
Preparing for the delve is a key tool in the Felltower PC toolbox.
Rumor-gathering (and reviewing), sage-hiring, equipment-buying, and pre-planning are all aspects of this. PCs who prepare are PCs who can solve problems that ones who don't can't.
Felltower is built on the assumption that it is repeatedly visited, and that PCs will prepare with the right gear, pour money into the hands of sages and old-timer former adventurers, and so on in order to learn what they can.
Equally, though, the main elements are designed to be things the PCs find out through hard-won experience, clever action, and specific attempts at actions. The players, more than their PCs, determine success or failure with preparation.
This is a somewhat tricky tool, given the way GURPS works. The PCs have knowledge skills, and it can be tempting to set everything to a roll and ask the GM, "What does my guy know?" and then try to follow up with more rolls against the skill when that doesn't seem enough. I try to solve this by giving a general, "here is what you know" roll, then leaving the rest n the PC's hands. It's tempting as a player to use knowledge skills as a chance to pump the GM for a hint about what to do. I attempt to cut that off after an initial wash of information to set the scene.
As such, this is a middle-grade tool. It's important, but as long as you've done the basics and play cleverly, it's possible to succeed without pouring money into knowledge and preparation. That said, there is a minimum you must do - buy rations, lay in healing potions, recharge power items, etc. - to succeed at all. Having done that, the extra is extra - useful but not an impediment to success if you forgo finding things out before you go instead of in play. It's not that Felltower is forgiving, just that players tend to resolve more through play than through preparing for success.
Risk-taking is an important tool in the Felltower toolbox.
You cannot get rich in a megadungeon, especially mine, without taking risks.
That's worth repeating with emphasis:
You cannot get rich in a megadungeon, especially mine, without taking risks.
Sometimes you need to take a flyer on negotiations and trust the potentially untrustworthy. Sometimes you need to engage in a risky fight. Sometimes you need to pull something, push something, touch something, or stick your hand into something. But equally you need to resort to this when intuition and experience and trial and error tells you to do so. You can neither touch everything or slink through the dungeon touching nothing and get what you want out of it.
Risk-taking is the most critical tool in the player toolbox. Even more so than combat, in a game designed to have lots of combat. Even more so than magic, in a game explicitly about a magical world. Even more so than negotiation, in a game where that's threaded into play to ensure it's worth doing and expected behavior. If you take too many risks, you'll pay. If you don't take enough, you'll lose out. Balance is critical here, but it's the fundamental basis for the other tools discussed above.