Could someone explain vampire castles to me, please?

[Makavcio]

So, I've read all I could in civilopedia and various wikia pages but I still cannot comprehend mechanics behind the Sanguine Pact. Let me ask a few questions.

1. The Sanguine Pact is supposed to be unlocked after clearing a barbarian camp. Other societies unlocked for me right after meeting their requirements (sending an envoy, finding a natural wonder etc). Alas, I was able to join Sanguine Pact only in medieval era after my ~10th cleared outpost. Is there a special way to do this, or is there a % chance of this triggering?

2. What are the conditions for building vampire castles?

2a. How many castles can I have? I wasn't able to build any until Renaissance, even though governor tooltip clearly states that by medieval era, I should be able to have 2. I had all necessary governor promotions purchased. Now, I'm in Industrial era, tooltip says that my limit is increased to 3, but the option to build the castle is grayed out. Nota bene, two of my vampires have "2 builds", one has "1 build". Wikia states that they all should have equal number of charges.

2b. Where can I build castles? Tooltip and civilopedia/wikia entries say that pretty much everywhere in my territory, and in neutral territory. Still, I was unable to construct more that one so far. Is there anything blocking me, like floodplains, (rail)roads or neighboring features/districts/improvements? I wanted to build my 2nd castle in hills for insane production boost, and my 3rd one in even more wacky flood plain with food yields over 10 per tile, but I have no option tu build the castle.
I'm trying to build on plots without improvements (only railroad). So far, I was able to build one castle in a flat desert tile only.

3. When do vampires get their bonus combat strength? I used them to kill a few units but nothing happened. They stood next to a unit killed by crossbowmen and stil nothing happened. Do I need to mele-kill something next to vampires, and not by vampires themselves? Their combat strength evolves together with my most powerful unit but they don't get the +1 for killing units.

I'm simply confused. Everything about the Sanguine Pact seems rather straightforward but nothing works as I expect it to.
I'm playing without any mods that would influence game rules in any way. The game is up to date. Other societies worked as expected.

 

[AriochIV]

1. It's random; you just got unlucky.
2. I don't know of any restrictions on where you can build Vampire Castles. You should be able to build the maximum number stated in the governor promotion description, provided that the Vampire still has build charges left.
3. The Vampire's base strength doesn't increase with kills. You can see the bonus strength in the combat preview when you have the Vampire selected and mouse over a potential target.

 

[Laurana Kanan]

1. The Sanguine Pact is supposed to be unlocked after clearing a barbarian camp. Other societies unlocked for me right after meeting their requirements (sending an envoy, finding a natural wonder etc). Alas, I was able to join Sanguine Pact only in medieval era after my ~10th cleared outpost. Is there a special way to do this, or is there a % chance of this triggering?

You only have a base 70% chance. This chance is further modified by the number of Civs that have discovered that SS and the number of Civs who have joined that SS on your home continent.

2. What are the conditions for building vampire castles?

2a. How many castles can I have? I wasn't able to build any until Renaissance, even though governor tooltip clearly states that by medieval era, I should be able to have 2. I had all necessary governor promotions purchased. Now, I'm in Industrial era, tooltip says that my limit is increased to 3, but the option to build the castle is grayed out. Nota bene, two of my vampires have "2 builds", one has "1 build". Wikia states that they all should have equal number of charges.

2b. Where can I build castles? Tooltip and civilopedia/wikia entries say that pretty much everywhere in my territory, and in neutral territory. Still, I was unable to construct more that one so far. Is there anything blocking me, like floodplains, (rail)roads or neighboring features/districts/improvements? I wanted to build my 2nd castle in hills for insane production boost, and my 3rd one in even more wacky flood plain with food yields over 10 per tile, but I have no option tu build the castle.
I'm trying to build on plots without improvements (only railroad). So far, I was able to build one castle in a flat desert tile only.

Castles should be valid to be built on all terrain types except of course mountains. They should also be able to be built on valid terrains with features, if those features can be removed (e.g. forest, jungle, etc.). Floodplains, for example, are a feature that cannot be removed so would not be valid. As far as build charges on the Vamps go, take a look here for some insight.

3. When do vampires get their bonus combat strength? I used them to kill a few units but nothing happened. They stood next to a unit killed by crossbowmen and stil nothing happened. Do I need to mele-kill something next to vampires, and not by vampires themselves? Their combat strength evolves together with my most powerful unit but they don't get the +1 for killing units.

For this, see my post here.

 

[enKage]

You cannot build vampire castles on floodplains, confirmed in my game.