Your problem is with this assumption:
I am confused because I've always been told that having numbers, cap/non-cap letters and special characters was essential for a strong password...
This statement is theoretically true, but false when it comes to how people actually create passwords. Simply replacing an o with a 0 and an a with a 4 does not make your password stronger. At best, the new password has equivalent strength. In reality, it is likely weaker because these substitutions are so popular that an attacker may try 0 and 4 before even trying o and a. Similarly, capitalizing the first letter is so natural that it doesn't add anything to the security. And adding a digit at the end is also a popular choice that increases the password complexity just a tiny bit, even with the extra & character added. Even typos (such as troubador vs troubadour) don't throw off a dictionary attack by more than a tiny bit.
As an aside, NIST has recently moved away from this recommendation (as well as from regularly changing passwords and a number of other common password policies). In the real world, both served to undermine rather than enhance security.
Having digits and special characters only enhances security if you choose each character individually at random, simply because an attacker has to guess each character among 90 or so characters. If you only use letters, an attacker has to guess only guess among 26 or 52 characters (depending on if you use a mix of upper and lower case).
In the case of Tr0ub4dor&3, those characters aren't random. This password is likely trivial to crack with a dictionary attack. Most people have an active vocabulary of around 1000 words or so, educated people of around 6000 or so (in English). These 6000 words would likely include troubadour. An attacker with a dictionary attack will of course try all the permutations of the word. So that's maybe 100 different versions of the word "troubadour".
Further, an attacker would try the most likely permutations first.
So an attacker may well try "Tr0ub4dor" before even trying the base word "troubadour". This word may be number 3000 in the password dictionary an attacker would try (although after the XCKD, it probably moved to number 20), so an attacker would have cracked your password in a few thousand to a few hundred thousand attempts - which may be a few minutes to a few hours with a decent computer.