For a strategy game, the player is some sort of a ruler (king, space emperor, dictator, whatever) and there are assassination attempts made. How to make it not so lethal (one unlucky random roll and you get game over)?

My first idea is like this: During the first *successful* assasination attempt you get a message "you got lucky and barely survived". During the second you "decided to show less in public" (and get a permanent penalty to something). The third means game over. So basicly you get "3 lives" :D

Also, after any successful assasination attempt there are no further attempts made for X turns/hours.

For a strategy game, the player is some sort of a ruler (king, space emperor, dictator, whatever) and there are assassination attempts made.

What are the properties of an assassination ?

1. A game stopper, like the nuke in any other RTS, where you need some way to stop endless games ?

2. Should an assasination be un-counterable ?

3. Is it possible to detect an assasination upfront and you can try to counter it just in time ?

In short: the player will most likely dislike any sudden-death scenarios and he don't need any warning shots. The first time he will die to an assassine, he will try to avoid it next time. And at this point it is necessary to be able to detect an assassine attempt upfront and to be able to counter it.

Eg. one enemy is build up a large spy-network(pre-requirement for assassines) instead of a larger army, therefor the chance, that he will try to assassinate me it quite high. Best to counter it by building up my own spy-network to detect any assassine plans.

The purpose is mostly for mood (the feeling there are assassins after you *all the time*), it has no gameplay/game balance purpose (or at most it's secondary/supplementary). Also, the assassination is purely internal (your own people being after you - to take your place for example or because they hate you), not enemy empires/countries.

With a moderate effort from the player the game should never end by successfull assassination (yet the player will need to put some work to prevent it from happening).

To prevent assassination there are two methods (best if both employed at once) - bodyguards and not making too unhappy too many of your officials/henchmen or at least not all of them at the same time :)

I want assassination attempts be quite common (and known to the player), like you get a notice/report/event from time to time that your bodyguards caught an assassin, or your intelligence/secret police tracked a planned attempt and stopped it. In general, I want the player to be aware or ALL attempts made (no hidden assassination).

Well, I'm not sure if an assassine will contribute to the mood of a game. It is similar to a ghost appearance in a horror game, once you recognize, that it is not really a threat, the mood factor will drop immediatly. Nowadays it is even likely, that the player will hear from the ghost upfront (yes, I'm looking at you internet) , making the whole mood factor irrelevant.

I would try to connect it to some kind of gameplay feature, eg:

1. assassine as cause for some events

This goes into the direction of emergent game play. If an assassine attempt succeed, you are trouble. Eg you get hurt and are out-of-business for some turns leading to different internal powers trying to exploit it. The result could be a temporary chaos/anarchy (people dont pay taxes), riots (people damage buildings, killing an important minster) etc.

2. assassines as result of some effects

In this case you always caught the assassine and you get some information about its client. Eg the merchant guild dislike your decision to increase the tax on some goods, you can either try to execute the client, which have an exemplary effect on other factions, but you loose some important connections to the merchants. Or you could try to pour oil on troubled water to avoid getting in more trouble with the merchant guide.

A lot of game recently have you playing not just one ruler, but a dynasty of successive rulers. This makes it possible to lose your "life" to assassination without the game ending. In order to adjust the mood, all you have to do is decide the after-effects of an assassination. For instance, if it serves as the impetus for a popular uprising against a hated line of rulers, it could still lead to a game-over scenario, but at least the player would have the chance to get things back under control.

The implementation of assassins in total war is somewhat similar to what you're talking about. you might want to take a look at it.

Most of the games I've played were the ruler can die at any time have you carry on playing as one of their descendants or relatives.

If that not an option then maybe you always survive the assassination attempts but either lose time the game skips ahead 3 months while you are recovering. Or you could acquire a negative trait of some kind. Maybe you become paranoid, erratic, or sickly.