Alliance victory too easy?

Probably yeah, on Easy/Easy

Alliances depend on the power rating difference between two people. Try bumping up the difficulty until the AI is at or above your power rating when you research the techs and it should be much harder to convince them. (And you'll see them doing more interesting things in general)

I wouldn't say that easy is supposed to provide a challenge anyway, rather to just play for fun.

Victory is as easy or difficult as you wish to make it, given the settings.  And even then, there's such a large degree of randomness in the way a new world's resources and topography are configured, you could still find an easy setting more difficult than you suspect, and a hard one easier than you might have thought.

Try playing at least on Expert and play a kingdom against only empires, or an empire against only kingdoms.  See how quickly you make an alliance then.

Yes it is too easy even on Expert... once you get a big lead, everyone will ally with you.  Game over.  Brad has programmed the AI to give us a +5 diplomacy rating (You are dominating me) and that's probably too high.  I don't want to take the fun out of it because creating allies should be a viable and doable strategy to win the game.  I also like it when I start getting bored with the game I'm in and don't want to finish mopping up.  I guess the best suggestion would be to turn that victory option off if it's too easy.

That's only because you and I have been playing the game for so long. Most people don't know it that well, and a relatively new player like the OP would probably have a difficult time playing a kingdom against three or more empires. on expert.  The likelihood of all three ganging up when one goes to war with the kingdom is strong, given how unnuanced diplomacy currently is, and it's tough to out-maneuver three opponents trading tech points and fielding troops.

They're paying you to ally because your power rating is far ahead of theirs - so the AIs are basically recognizing that you could stomp them in a war even though you choose not to, and thus they decide to give in and cooperate without forcing you to kill them. The alliance victory makes sense in theory, but there are some issues with it.. I would say that these issues have more to do with the power ratings and diplomacy in general, though, not the alliance victory per se.

Problem one is the power rating calculation is woefully inaccurate; it's weighted heavily for population and sheer number of troops, without caring as much about technology or equipment or experience/levels. It thinks enough (and not even very many) naked club-wielding militia > a level 30 sovereign with the best quest gear. Meanwhile the combat system favors the sovereign - and that's not a bad thing imo, the power rating just needs to realize it. Problem two is diplomacy in general hinges too much on power rating, which would be a problem even if it was accurate. AIs will never consider, say, teaming up against a stronger player, they just gang up on the weakest player and effortlessly give in to the strongest player.

Anyway as everyone has said, the easy way to circumvent these issues is to find a difficulty level where the AIs can roughly match your power rating. That way none of the diplomacy wierdness associated with dramatically different power ratings comes into play, the diplomacy AI is actually pretty sane when power is equal.