I have successfully made climbing ladder ability. The toughest part is to define where ladders would be. The other stuffs such as climbing down and climbing up animation + transition are easy to make.
I could share you how but TBH I don't think how I define ladders is best way. I need to research better ways.
Once ladders are defined, the next step is to check for it in WALK and UP animations. I only use these 2 since players must be walking in they want to climb ladder.
Though from my experience in implementing this, climbing ladder from below is easier than climbing from top of ladder. Not because of script problem but more to detection time.
I guess that's because naturally ladders are placed on walls so when players walk to ladder to climb up, they are blocked by walls so they won't miss the ladder but when climbing down from top, nothing blocks player so they usually miss and fall down instead.