Aerial Rush is what I think that's what it's called but I don't want to spoil myself while the emergency maintenance is still happening to be more accurate, Edit: It's called Aero Runner and I will say that it's definitely in South Aelio. I think it's on a mountain named Mt. Piccolo or something close to that to be precise. Edit: Mt. Piccolt. Anyway the reason I want to recommend this tower over all the other ones you can find is because I believe this is the more beginner friendly tower to start with since this is actually an Obstacle Course.
All you have to do in that tower is glide to the exit. The only things that can hurt you in that tower is one enemy that shoots and the bombs that are scattered around in the middle and the last phase of that course. I'm pretty sure those traps won't kill you but instead only make you almost dead if you get hit by them instead if you're taking it on at Lv. 5 before you learn about Sub-Classes, unless the bombs do percentage based damage and then all you have to avoid is the easily dodgable shooting enemy.
Towers and Cocoons don't restrict your entrance to them by your power level or level itself so you can enter them just fine. Just be prepared to not have a good time if you're not close to their recommendations. Anyway this Aerial Rush Tower should be the easiest one to do to get your 4 skill points from and all you have to do is reach the exit. Good Luck.