Vikings can't swim? Is there other entrance than the water?
Did legion were so idiot letting being drown by a click from outside their fortress?
In the night, what's in this big cube? broken, nope, nothing in. In the day I saw it let the water running in the fort. Who wanted to prevent them drinking?
I imagine there is a solution, because the quest is stil live. Or it's broken.

You have to get into the sewers, to the temple. You don't go in via the water. Hint: Romulus and Remus.

If you've drowned them, then you cannot visit the legion inside anymore. If that's the case, then the quest should have failed (and not be in your quest log anymore).

However, if this quest does involve Romulus & Remus (as per the hint above), then you shouldn't need to go inside for it, and it should (hopefully) still be possible to complete.

It's been a while (many patches ago) that I played through this, but I do remember the legion-related quests being more bug-prone back then.

Okay thanks, I'll search trying to involve those R&R (didn't know them in the game). Or the quest is bugged.
EDIT: Oh, they are surely among those who suddenly did attack me in the fort, I didn't know why. I killed them all because I had not offended them. Then I saw about this water affair... What badly done quests here.
EDIT2: Then I met guards on a bridge, they follow me, I don't know their purpose, what to do with them.

It's possible you've done things in a non-optimal order for this series of quests. It shouldn't be a big deal, it may just mean you have less people to send to your island. Which equates to missing out on a small number of Power and/or Prosperity points, but unless you've been failing quests left and right both of these can easily be made up via building improvements without limiting your options later.

Basically, the "Roman" faction is made up of two main sub-factions, plus there are Christian zealots near the bridge, plus some refugees across the river surrounded by some small woods. Looks like you've inadvertently helped the Christians wipe out the Romans - which is a perfectly valid resolution to the quest. If the refugees are still around you may still have some options with them.

There are also the three stooges (er, craftsmen) who IIRC this particular quest was for - if you've drowned or otherwise wiped the Romans out, then go back to talk to them - you should still have some options if I'm not mistaken.

Nope, they fleed. Nope, they're still just waiting for their money, I can't say them they did a so bad work. They stare here instead of having consolidating the barage I had mistakenly broke.
They don't deserve their money, but I can't say them. They should have did a big warn "DANGER, DON'T CLIK HERE!"
I'm sure the stooges are the devs of this quest.

Dang, looks like they still haven't fixed this properly. If you're really concerned about that quest I guess you'll have to start over from when you arrived at that location (assuming you have a save that far back, and don't mind replaying those sections). Otherwise you should be able to safely ignore that quest (helping those guys out allows you to send them to either your village or your island, but they only contribute a small amount of Prosperity iirc).

Come to think of it, I think I probably restarted that whole section a couple of times when I went through it, until the sequence made some sense to me. The best way for this group of quests is to talk to as many quest-givers as you can when you arrive, but don't actually perform any actions until you're sure of what you want to do. That way you'll get a much better idea of what is actually happening on this map, since it's definitely not clear at first - and it's all too easy to accidentally start down a path and then cut yourself off from quests and their rewards (as you've found).

IMO one of the issues is that those three craftsmen are located in such an out-of-the-way place, that the first time through they'll likely be the last people you run into. They really should have placed these guys to be on the way to the main entrance or something.

I ignore the game, waiting for the good patch. I entered the map just on them, so it was not the issue for me.

Just a bump. Sad this quest killed me the game, excellent until this.