In the gameplay of Thick as Thieves, while stealth and subterfuge are the standard way of going about things, combat is not absent as a possibility. However, it definitely plays more of a secondary role within the gameplay and tends to be mostly defense-focused, instead of the attack/offense approach common in most roleplays and RPGs.
Defensive combat denotes any combat, armed or unarmed, in which you engage an adversary/opponent for the purpose of protecting yourself or others (e.g. co-workers), buying yourself some time, or trying to create a distraction/ruse. The point of defensive combat isn't necessarily to defeat your opponent completely, but to gain the upper hand and outwit him/her at least temporarily. Given its very nature, while professional thieves have learned to be rather good at defensive combat, their skils in more offensive fighting styles can vary wildly. Usually though, a thief is not a fighter at heart, and certainly not a warrior or soldier.
1. Like it or not, you're a thief, not a fighter. “Worship your life, not the sword.”, some wise Melzan once said… Even if you have good skills at brawling and weapon wielding, that doesn't necessarily guarantee you'll get out of every single scrap. On the contrary… So, be a Tactician first and foremost, and use your common sense. Be a Fighter only when all other choices have been depleted.
2. As an extension of being a Tactician, always pay attention to your surroundings and try to learn “the lay of the land”. (Yes, that also counts for indoors, as you'll be doing a lot of traversing of both exteriors and interiors in your mostly urban adventures.) Use the terrain or the surrounding spaces to your advantage, whether it be for stealth, defense or an open confrontation. Weapon wielding skills are important, but they are certainly not the sole thing you will have to count on in a tricky situation.
3. If you truly cannot avoid combat, at least try to avoid being forced to fight in a situation or space where you're clearly at some sort of disadvantage, particularly a major one (e.g. you're tired, you're already wounded, your weapon would be ineffective in the space you are in, there are overwhelming numbers of adversaries, etc.).
4. The size, weight and general wielding properties of your weapon always matter. Why would you use a polearm for self-defence in a room where you don't even have the space to swing or thrust with it ? A shorter weapon would suffice in such a situation, and so on… Also, what many glamorous tales of rogues forget to tell you is that would-be-thieves often forget to learn how to draw their weapon quickly but effectivelly. The opponent chasing after you won't pause to stand in place as you clumsily try to unsheath your sidearm. He'll use every second, every moment misspent by you to his/her advantage. Don't ever give the opponent such an embarassing crutch for cornering and beating you up more easily.
5. “Too many dogs, the hare's death…”, goes an old saying popular in many countries of Aporue. Long story short, unless you have at least a semi-competent team of co-workers or co-fighters to support you, forget about taking on more than one or two adversaries at once. We're talking about common, humanoid adversaries here. Things get even more uneasy if your opponent is something much bigger, stronger, or generally far, far more dangerous, even if you encounter it in singular numbers. So, if you're alone and have to rely on your own wits, get into a brawl or skirmish with only one person/being at a time, not several at once.
6. It's pointless to revel in needless violence or even the killing of your opponents, especially if they are sapient beings and are regarded as citizens of some sort. Bad enough for you that you're a thief by choice, no need to pile up on the possible repercussions for your more dishonourable actions… Though you might argue that you can simply hide the evidence of your crime, doing so wastes time and evidence can be a dangerous burden to you in case it would resurface. So, even if you are not above committing it from time to time, avoid killing whenever possible. As an old thievy saying goes, “Reckless noisiness and reckless violence are two of the greatest and vainest luxuries.”.
7. Sometimes, even if true stealth is no longer an option, you can find an alternative to a direct confrontation by creating ruses, decoys or distractions on the fly. It's no surprise that the inventory of professional thieves (and even the Melzan police force) contains a surprising amount of smaller weaponry that has no other purpose than to stun, daze or confuse an opponent. The many various types of trick explosives are the most famous example of this, but there are plenty more gadgets whose sole purpose is to turn the tide in your favour, even if it's only for a temporary, fleeting amount of time. But in a situation where every single second and every single move can be of the essence, you'll learn to cherish the option of having a middle ground between sneaking/spying around and brandishing whatever typical weapon you might have.
8. Even while you fight, you don't have to throw stealth out the window or blow your cover completely. If there's still a way to avoid alerting too many people about the fight you're having with your opponent, then by all means, exploit such an opportunity and try to keep the fight unheard and unseen. Making a big, showy, loud fight will only attract more attention and make things gradually worse. So, all in all, try to be subtle, even when you're parrying, slashing, punching, kicking or bludgeoning someone (or something).
9. You can have all the brute strength in the world, but if you don't back it up with plenty of dexterity and learned skill, you might find yourself a goner surprisingly fast. There is no such thing as a single good quality or single good skill that will make everyone into an invincible warrior. Every single individual is a collection of weaker and stronger aspects governing one's combat capability. You can never have only advantages, without having at least a few drawbacks.
10. “Surely, women are always weaker then men in a fight ?”. Erm, no. Everyone is an individual and one's sex or gender alone doesn't define whether he or she can be an effective or even formidable fighter. Just like it doesn't particularly matter whether a practitioner of a thievy specialisation is a man or a woman, so too does it not really matter whether a combatant is male or female.
11. Unless your opponent is somehow incapacitated in a way that he can't move at all, there is no such thing as “pulling a foolproof move that cannot ever be countered by the opponent”. Aporue is a place of wonder, magic and plenty of weirdness, but even the greatest fighters to have ever lived will tell you sincerely that you can't create a “foolproof move that always works” out of thin air and wishful thinking. Well, you can, but don't be surprised when the day comes that you run into someone who counters your “super-special move” with absolute ease. (And then makes mincemeat out of you. How embarassing indeed…)
Though there is a great variety of handheld weapons, there's really no such thing as “weapon levels”, from the weakest to the most powerful weapon, etc. No. Instead, you will have to pay a bit of attention to what exact type of weapon you choose for certain types of combat (usually of the self-defence variety, of course).
For instance, if your character acquires a stabbing-focused dagger (stilletto, poignard, etc.), it will obviously not be of much use if you get into a situation where a more fencing-capable weapon would be a better choice. Or, let's say you've got a small rapier and it's pretty good for light-footed fencing of the slash-slash-stab-stab variety, but is certainly not much of a chop-chop weapon.
Then there's the fact that wielding a straight sword and wielding a curved one (a sabre, basically) also requires you to use it in different ways and on differently armoured enemies, if you want them to be effective. In short, whichever particular type of weapon you choose (from whichever main category), it will have its logical drawbacks in a potential fight, based on its OTL historical usage and on physics-related common sense.
Something of a general rule for carrying weapons, gear and devices: Your character can carry a scabbard on his back all he/she likes. I'm fine with that. But he/she won't be able to draw his/her sword from it without putting it into hand's reach. Drawing swords from the back might appear cool, but it is needlessly impractical and is very hard to achieve even with a shorter blade. So it's out. Carrying can stay, but drawing, no. For immediate drawing, you'll have to wear your melee weapons on your character's belt.
Some inspiration here.