It's information that is in addition to another idea/argument that was said, rather than an additional item. Such as, "Why are you riding that horse when you should be in school?! Besides, the horse is mine!" as opposed to "The pig is mine. The horse is also mine." I'm not sure I'm explaining this very well, but I hope it helps.
Spanish uses the upside down question mark and the upside down exclamation point at the beginning along with the "regular" marks at the end. It's kind of cool because it gives you a heads up as to how to say the sentence to begin with.
Anyway, I have a Mac and the upside down exclamation point is alt/option + 1 and the upside down question mark is alt/option + shift + the backslash/question mark key (I had to google to figure it out.)
Yeah, it is cool. In English you sometimes can't tell a sentence is a question until after you have gotten deep into it or even all the way to the end when you finally see the question mark. Stupid! Spanish tops English in this instant by far.
I have been trying to start writing Spanish sentences that are questions by beginning them with the "¿" character but so far I have only been able to remember to do that just a few times. Darn it all. And it is easy to do it too on a tablet PC. It's right on the keyboard.
In Windows, you can install multiple keyboards. Use Google to find out how. But once you have multiple keyboards set up, you just alt-shift to switch between them. To make the upside down exclamation point with Windows' Spanish keyboard, you use the plus-equals key. There are keyboard diagrams for the alternate keyboards on the web. I printed one out for reference until I got used to where the new keys are.
It's "besides" - "beside" and "besides" are different words. And it means what it says, "Besides, the horse is mine!" (or "Aside from/In addition to (whatever has been established in the previous statement), the horse is mine!" or "Furthermore, the horse is mine!" (Duolingo missed a comma here)
"On top of that", "more than that" are common phrases in American English universally accepted for this context. They need to be added. Duo has done a great job they just need to continue adding in the American English section because American's want to say things 50 different ways in order not to repeat or appear as if repeating.
So in the English version, the comma indicates a brief pause ("Besides, the horse is mine!"). Since there is no comma in the Spanish version, does that mean it would be unnatural for you to have a slight pause after "Además"? I can't tell if Duolingo intentionally avoided placing a comma there, or just decided not to include one, but you theoretically could include one.