If you are thinking about joining a guild, the first thing you need to keep in mind is that any guild is a profit-oriented commercial undertaking. Otherwise, it simply would not exist. However, having to dip into your purse deep and often during your training is not even close to being the most unpleasant thing...
At the guild, training takes place as follows. For a basic fee, the guild undertakes to teach you all skills that meet the guild’s minimum entry requirements “free of charge”. Say, the Warrior Guild requires its recruits to have not less than eight Strength, six Dexterity and Constitution, and five Intelligence. These values of primary characteristics determine the level of corresponding skills which you will mandatorily master.
Fencing relies on Dexterity. The Guild's entry requirement is six Dexterity. Therefore, for a basic fee, you will be able to train your Fencing skill to +6. Meanwhile, Hammer Mastery relies heavily on Strength. Which means you are guaranteed to raise it to +8. Want to progress further? Well, the bad news is that you will most likely have to raise your Guild Level first. The same applies to every extra skill.
The thing is, you can't raise a skill above the basic skill level at the Guild. Further progress requires real combat experience, which is obtainable during actual missions, and this is where the Guild Mark comes into play — a unique mental artifact that is issued to every Guild member. It keeps track of your performance during training, and then the Guild Masters perform a thorough analysis so as to adapt your technique based on accumulated experience. This is an extremely challenging and thankless job, so be prepared to face a tough row to hoe for the benefit of the Guild.
Typically, this includes quests, which thankfully depend on your Guild level. Working as a handyman in a forge is a piece of cake. Protecting a flock of sheep from wolves overnight is not a difficult task either. However, high-level Guild members have to grapple with a nightmarish routine of "Go I Know Not Whither" and "Fetch I Know Not What" type quests. Incidentally, failure to complete a mission lowers your Guild Level, which, depending on the content of your Guild Mark, can deteriorate not only your mood, but also your actual combat effectiveness...
Having high Intelligence will save you a lot of money. If your Intelligence value exceeds your current skill level, you have every chance to progress naturally, even if it takes more time. Let’s go back to the example with your Fencing skill. If you have mastered the basic skill level at the Guild (which is +6), and at the same time you have ten Dexterity and nine Intelligence, then you can independently raise your Fencing skill to +9. But to raise it to +10, you will need the help of the Guild. However, it is obviously impossible to raise a skill above its primary characteristic value. Your Fencing skill level will never exceed your Dexterity value, unless your resort to some high-class artifacts.
Yet, no matter how smart you are, it’s next to impossible to outwit the guild — they will still find a way to rip you off. Over the centuries of their existence, the guilds have accumulated vast experience. And some of that experience is nested into your Guild Mark, depending on your Guild Level. Even with the same skills and weapons, a rookie is miles apart from a high-level guild member in terms of efficiency. How could it be otherwise, if the Guild Mark endows a loyal guild member with invaluable insight into the enemy’s strengths and weaknesses, even if it is encountered for the first time, right off the bat.
Needless to say, it can bestow knowledge about artifacts and high-level weapons that would otherwise be incredibly difficult to attain on your own. Want to know more about adamantine and its combat applications? Well, if you please, first raise your Guild Level to 50. Thus, you will attain this knowledge, sooner or later, one way or another. Another thing is that by that time you will be able to master the absolute majority of skills, while, say, an orc with innately low intelligence will need a Guild Level of 100...
Now, each guild has its own Guild Mark, and you can only wear one at any given time. Which means that the accumulated experience to improve skill levels will only be taken into account for the current guild. However, if you have already mastered Fencing at the Thieves Guild, you will be able to enjoy the same high skill level in other guilds, say, the Explorer Guild. Meaning, you will be able to use a dagger just as effectively against the familiar enemies. Should you encounter a new and unknown adversary, a higher level Guild Mark will most likely give you a significant advantage.
Although one can join several guilds at once, you would be well-served to scrutinize the desired skill set in advance. The simple truth is that your Intelligence value affects the number of skill trees you can master.
It is no coincidence that the Warrior Guild set the minimum requirement of five Intelligence — this is exactly how many specializations it offers: Attack, Defense, Physics, Archery, and Team Work. A total of fifteen Intelligence enables you to fully study the skill set of several guilds, another question is whether you have the time and patience for such an undertaking...
P.S.
Oftentimes, gold can ameliorate the issue with Guild Quests, but even with the most generous donations, once in a while, say in one out of five to ten cases, you will still be compelled to get down to questing in order to get the guild’s help you seek.
Therefore, here’s a piece of advice for you to heed — start saving gold!