Hi all,
this is just supposed to be a collection of useful tips that I consider basic gameplay knowledge valueable for new and experienced players alike.

As a requirement, you should be familiar with the complete guide and walkthrough in english made by Steven W. Carter in 2003: https://gamefaqs.gamespot.com/pc/561594-lionheart-legacy-of-the-crusader/faqs/25323

There you can look up certain things that I refer to.

One of the most important basics to keep in mind:
before you open or even attempt to lockpick a treasure chest, make a quicksave.

This way, if you fail to lockpick the chest, you can quickload and try again.
How often should you try?
If you get consecutive fails, try 5-10 times, if it stays the same, try using a potion of master thievery, which boosts your lockpick by 20 points. Don't worry, you do not lose the potion, because you can easily quickload if it fails. Then, if you still can't open the chest, leave it be and get some thievery equipment that boosts lockpick. If it works however, and you drop something good, it was worth the potion. Do not hesitate to use a purple pot if you can turn it into value.

Secondly, drops are not fixed until you open the chest. So if you drop the usual garbage that nobody wants, simply reroll the treasure chest by quickloading. Rinse and repeat untill you drop something that you can either use right now or later, or that you can sell for good money.

Necklages of carnage (elemental type/weapon type damage +10%) sell quite good, especially when they have more than one element damage boost.

Attribute items are always a first pick and if you get one, you should stop rerolling the treasure chest, because you can either keep it to get to a certain value for specific events that check your attributes, or sell it for lots of money.

There are perception checks for certain quests, a luck requirement of 10 luck to enter the old shipwreck secret area, several strenght checks for an intimidation dialogue choice (if you try to convince Quinn to join the dark wielders, but as a mage have the mininum 1 strenght, attribute items can help you boosting your attribute to the neccessary value. Also, alcoholic beverages avaible at the tavern boost strenght aswell) and requirements to be able to get certain perks. E.g. Disease Ward requires 5 endurance, but if you are a whimpy mage with 1 endurance only, you'll need these attribute items to boost your endurance to 5 temporarily, then get the perk, then unequip the items again. They can also help getting more hp/mana from levelups if you keep an eye on your exp and equip them shortly before you levelup, just as you would do with voodoo/mastery gear.

If you get something that could be worth something (1000-9000 gold, depending on your barter skill), but you want to keep on rolling because you either feel lucky or urgently need that load-bearing+speed+adder boots, make a normal save and name it loot_item_effect, so you know what that loot was, then keep rerolling the chest by quickloading. If you get nothing good, you can always choose the best loot out of your normal loot saves.

There is no guarantee that you'll get the item you want. I've spend 30 minutes rerolling a chest once, had to settle for the next best valuable item to sell for some gold.

If you are new to the game and/or don't know certain maps so well, scouting is what you need to do.

What is scouting?

Scouting is basically making a quicksave after entering a new map, then drinking a potion of master thievery, then exploring the map in search mode whilst ignoring all enemys, just looking for traps, secret doors and hidden treasure, aswell as counting enemies and remembering how many enemys of what type stand at which place. Then you quickload and prepare according to your newly gained information.

Why should you scout the map?

First of all, traps. They hurt when you walk into them, especially in combat, and you'll miss out on sweet sweet exp if you do not discover them. (same thing for secret doors and hidden treasure)
Secondly, to get a hunch where to go and what to do where.
Lastly, to know what kind of enemies will be attacking you, so you can prepare in advance by walking back to a merchant and buying fire protection scrolls against those nasty lava trolls for example.

After the scouting you can avoid traps or use purple pots to directly discover them since you now know where they are. Usually, you'd want to first kill all enemies, then use a purple pot to discover all hidden treasures so you don't waste your pot time by fighting off creeps. However, at some maps you'll run into unavoidable traps paired with enemies, especially in the sewers, where you need to know where the trap is, so you can lure the enemies over to your side first, kill them, and then defuse the trap.

Chosing perks is very important for charackter building, even more so is chosing the right ones.
You might be tricked into chosing that 15+ ranged combat on level2, but let me tell you, that is pretty stupid. There is lots of exp to gain in this game, means you'll get quite some levels and there by alot of skill points. Don't waste your perks for things that you can get easy otherwise.

The amount of perks you can get is limited, so try to plan wisely instead of choosing a perk just because it suits your class/chararckter. Perks that profit your char early on, such as swift learner or educated should be taken first above all else. You can add that undead glory perk later without missing out on something, however it does not make sense to pick bonus-exp-perks in the lategame when there is not much to get anymore.

Perks that give +X on a skill should be avoided. Favored skills give 2 points for 1 skillpoint spent untill you reach 77 points, so getting +15 from a perk will only be worth 7.5 effectively. It's best to use your skillpoints from levelups to push skills. You might consider using such perks in late-late game just to get the last 0.5% of power out of your build if you have nothing better to pick

One particular important perk is disease ward. no matter what your build is.
Disease ward lets you heal and beeing immune from disease type damage, which is used by "almost" everything in the game. Undead, some beasts and the wererats including sewer beggers and the daeva boss that deals lots of disease type damage.

Why disease ward and not venom ward? Because poison resistance is the easiest to stack in the entire game. Adder boots+20%, Dead of Night amulett+20%, snake rings 10% each, makes 20% if you have 2, total of 60% already, +40% from green pot/Purify Body buff = 100%. And you can stack disease ward (+110% disease resistance) with snake eater (+30% poison resistance).
So yeah, it's easy to stack poison resistance, but not disease resistance, so that is a choice of priority. Mage players may want to use attribute items and beer to boost their endurance to 5 to be able to pick this perk.

Here's a bug you need to know:
if you intend to choose either disease ward or venom ward as a perk (lv11+),
unequip all equipment that gives resistance to disease or venom and wait for all resistace buffs to end (green pots/Purify Body buff), else your charackter will get bugged.

Disease/Venom ward will give you +X resistance once, where X is the difference between your current resistance and 110%. So if you have 40% resistance thanks to Purify Body, then select the perk, you'll get +70% resistance and if the 40% buff ends you'll only have 70% resistance instead of 110%. That's why it's important to wait and unequip.

You can make the game easier (but lose out on a whole lot exp, so I only recommend this for new players) for every map or cave that you can get into without any quest requirements.

What does this do?
Easy moding will give you weaker enemies on all maps before Montserrat.
It's basically a bypass to the games auto-balancing of difficulty.

How does it work?
You basically need to stay low-level and enter each map and cave once, so that the map spawns low level monsters. Then you leave and return later when you have a higher level to prey on low-level monsters easily. This will cost you quite alot of exp however, that you don't get because there are not as many strong mosters around.

This should only be done by unexperienced players or partially for speedruns on certain maps only. (making Unholy Oubliette or Troll Pit easy can save a lot of time if you are one of those speedrun streamers/youtubers)

Sometimes in the game, certain items will tell you that people will react negatively towards you.
Other sources, such as guides, may speak of - x Karma gain for certain events, such as killing merchants (don't touch their treasure chests or they will attack you) or cooperating with Daevas.

This affects merchants not showing you their "special offers".
It's bascially a second trading catalouge, like Weng Choi has 2 if you bring him a book.

Why are special offers so important?
Merchants are kinda like giant treasure chests that randomly fill up with awesome items and potions.
Having 2 trading catalouges will effectively double your "loot" and double your access to buying potions. So in order to get really strong items, you want to stay on the good side with all merchants.

For example, the merchant on the crossroads will give you the special offer option after you've wiped out the goblins, bandits and wasps from Estebans quests. These quests propably grant local positive karma. However, if you then posses an item that lets people react negatively, the special offer option will disappear again.

Do not sell equipment that could help you in certain situations.
Learn when to use what gear.
Learn what to look out for, what to keep, and what to buy.

Yes, you can play a pure necro with no combat or thief skills, but that is very stupid, because you'll miss out on treasure that gives you equipment (and oh yes, necros are equipment-heavy) and you'll deplete your mana with magic attacks, which you need to keep your summons alive.

You'll effectively only need necro gear when summoning, after that you should switch back to your combat gear. Same with thief gear. Get gear that boosts thief skills, use it for scouting and lockpicking, then switch back to your combat gear. Yes, it's a bit annoying to change gear 2 times each few minutes, however, it's the most logical thing to do if you want to get the maximum out of your charackter.

Especially necklages and rings are important, as they have no weight, so you can carry a wide variety of them to be suited for every situation. Even as a warrior you might want to use that summon undead ring from time to time, just to have another meatbag on the field, or to lure enemies, such as certain goblins or thiefes that will shoot arrows at you while standing behind impassable barricades.

Another point would be attribute items, even if you can't even wield them. Just equiping them and pushing your attribute to a certain value is enough to be able to perform various actions you could not do otherwise, thus my necro also got a belt that gives +3 strenght total (wish I droped that on my warrior), which is useful when trying to carrying all that loot to a merchant when you only have 1 strenght and get overloaded easily.

...there may be more tips comming if I find the motivation and the time for it, however I'd rather focus on writing actual guides on more detailed matters like how to spec/build certain classes, etc.

I hope you could take something useful from these random accumulation of words that only happened because my house-duck decided to walk over my keyboard. :)