Style question: would the party know (or be able to find out) about the gargoyles or the boiling water before deciding to make the climb, or would you introduce those complications after the climb began?You're conflating challenge and difficulty here. Getting from the ground through the tower's window is the challenge. The difficulty is low for a group that has at least 2 ropes and a grappling hook. It may be more difficult for a group that doesn't have these resources. And maybe they do, maybe they don't. Perhaps they used them in previous challenges and don't have them available anymore. In this case, the players prepared ahead of time, still had access to the resources, and then employed them - good for them! Will they leave this rope in place so they have a way back down from the tower in a pinch? If so, that's two fewer ropes to use for solutions to subsequent challenges. Play to find out if it matters.
Now, if you're playing in my game, gargoyles are likely going to come out and try to knock you off that rope. Or guards will be throwing down pots of boiling water from the top of the tower. Or something like that. This increases the difficulty of the challenge. But it still doesn't mean there's a roll, provided the players can come up with a means of overcoming these difficult situations while climbing with certainty. If their solution, however, has an uncertain outcome and a meaningful consequence for failure, then they will make a Strength (Athletics) check to accomplish the task of climbing while encountering a difficult situation as the rules state.
(Just curious, no judgement either way.)