Invisible buildings?

[alpaca]

Hmm I guess this would require quite a lot of rather complex scripting work including a background script and maybe some show_me events.

I'm rather adept at scripting myself so it's possible my ideas won't work, but I'll try to figure something out:

1. This is not possible like you want it. You can't edit diplomatic behaviour.

2. This is probably possible with a show-me script that triggers on CharacterTurnEndInSettlement.

3. This would be possible using a monitor_conditions checking DiplomaticStanceFromFaction and a monitor_event FactionTurnEnd that counts up a counter.

I'd suggest something different:
Check if a spy is in an advanced settlement. If he is, give the game some random percentage to fire a show-me.
This show-me would offer the player to recruit a special ancillary called "Wall Builder" for some amount of money.
You'd also run a background script that checks for every character on turn start if he is in a settlement; if he is, and has the "Wall Builder" ancillary, you'd create your building on turn start.
This would resemble the wall builder teach the locals how stone walls are built, or that he leaves some of his students there.
The clear downside on this is that you can only transfer this ancillary between spies.
You could also give the spy a special trait instead of an ancillary so that he becomes the wall engineer himself.