Yay let's use the fact that a player has a high perception against them. Do you also point out which flower in a field has the largest stamen, or the heights and weights of everyone in a village, the general size and consistency of dragon droppings ? a high perception would notice those things as well. Perception in game is for finding things that are useful, in the real world we all have our own filters to determine what is or isn't important in the game world you are that filter.
Right, well,...
I should of ignored it, and if the party proceeds up the road and can't handle the orcs, (and thus retreat back down the road) from the players POV, a 'magical' pipe would of appeared (above the path they had already traversed and 'explored'), with water gushing out, effectively hampering their escape.
I can hear the accusations now,...like, for instance, "What! We DIDN'T noitce?! I'VE got a 24 perception, there's no way I would of missed it!"
So they didn't spend any time in a bar or walking through the city at all ? a 24 perception can get you to see moss growing on a hole 3 hours up a mountain side but nothing lets you overhear conversations or rumors ? If the plumbing is extraordinary then I would sure mention it. "Hey you may not want to be on the road at midnight cause this happens" I assume the vatican of today has the common plumbing of today or something similar. If the type of drain found at the citadel is "common" then there is no reason it should have been something so out of the ordinary that it raised any "flags" when noticed. If it is extraordinary then it should have been mentioned by someone who had noticed it prior.
Unless of course, you've lived with it your whole life,..it's just water being flushed out a pipe. If your going to talk to a tourist, you probably:
a. get them to buy one of your ales at double the price first.

b. Talk about the things that make you proud, like the strength of the battlements, the fact that the citadel trains the legendary dwarven paladins and the like.
c. Not even think about the waste water.
d. Or simply (at best) not talk at all to the Half Orc and his friends,..since Orcs are attacking once again.
Or the fact that historically this maneuver has been used in the real world by countless civilizations ? Using the sewer system as a form of ingress/egress is a brilliant strategy both in game and in the real world. If you are going to ask them to "turn off" every experience they have had or read about in the past then better communication will be paramount.
Yeah, something like, "Don't bother wasting your time, you can't get in."
Subtle enough for you? Now, if you go forward, mind the turn, the railway veers a little to the left,..oh, and get your weapons ready,..you never know what's around the corner

,..actually, let me clarify any misunderstandings you may have,...your going to get ambushed by nine orcs, but don't worry, you'll be ready, and 5 of them are minions,...look for the little minis,..yes the grey one,...don't worry, I'll mark them for you."
Oh, what Joy.
Repeatedly by insulting the two that went up the pipe and refused to address what historically both in the RL and in gaming worlds has been a great stealth tactic. I fail to see how "hey the 5 of us should just walk up the main road pound on the main gate and deal with whatever answers" is the more reasonable approach
OMG, your right! I've corrupted the accuracy of pipe design 101 both historically and in gaming worlds. How could I of overlooked such an obvious flaw?
The idea by itself is fine. But, coupled with,.. 3 'game world hours' crawling along in single file in a wet, slimey, pitch dark and verticle pipe which could be flooded anytime, resulting in me being flushed over the mountain,...hmmm,..I'll walk on the road thanks.
But they are free to explore it.
Two players saw an atypical and stealthy way to perhaps gain entrance and have the upper hand/surprise on whatever was waiting in the citadel
So now its an atypical entrance,...damn I'll have to call the architect again.
three wanted to blindly follow the road up to the part where the enemy had defenses ready and likely should have seen them coming and have been prepared.
True.
Were I in your party given the information you had given me, none of which appears to have included (to the players at the time) anything about high pressure water shooting out this hole, but did include "it will be a long difficult trek" which in my mind translates to "they won't have bothered guarding this area" I know precisely what I would have done.
Hmmm,..5 pipes adjacent to one another,...the stone on the road worn smooth by years of water erosion, no side barrier on this section of road, metal filings and slag residue found in water pockets, knowing weapons are made up there, snow capped mountains, a RIVER originating from the mountain underneath them, wet slime covering the pipe walls,...and you deduce that it must be perfectly ok to climb up the mountain side to the pipes, split and weaken the party, burn 3 hours in the trip, knowing you wont get flushed out because 5 PIPES only handle small volumes of water,..and find a trap door at the end of it, letting you in, because all architects include that in their design, don't they?"
Thats the information the party had.
A few times I saw you use "for all intensive purposes" I was pointing out for your edification that the phrase is actually "for all intents and purposes" not being snarky about it just thought you might like to know the difference. The monopoly reference was to a post after yours because well someone brought up free parking.
Oh! Thank you. I stand corrected.