In some sense, what is done in MP is best though I don't agree with CragMak that you shouldn't ever build siege before cannons. Cats are kinda vulnerable, but Trebs survive pretty well and a few can take down a castle/hill city in just 2 turns. XB's will sit there forever and get eaten unless you happen to have range, and even then cavalry will charge in to kill your nice XB's as you are taking all those turns unless you screen them. And if you screen them you need a good screen of at least 3 melee which are often eaten alive by the power of a castle city and enemy XB encamped. XB's are really more effective vs. units which they literally chew right through. I would use my melee and XB's to fight off the waves of units while I snuck in with trebs that I screened. So much easier. The XB/knight strat is nice and all, but if you don't have good balance you won't win a Deity domination. I've tried XB's, even logistics and ranged ones and it isn't the end-all. AI throws loads of knights at you. Builds forts. Hides cities screened by hills and forest. Then I snuck 3 siege in, lost 1 and the city fell. It was by far the better choice in that circumstance. If you give the AI or human 4 extra turns they'll just use them to reinforce the position. So siege is good for taking down strong points speedily, then it's your turn to camp and heal while the AI desperately throws units at you that your logistics XB's can mow through. See how that works?
Now, XB's are the most versatile and the best for defense as they chew up units better than siege. Siege are solely good for taking down strong cities. So build more XB's then a few siege later when you intend conquest, maybe treb time. No sense waiting for cannons if castles are already up in your enemy's lands, and they should be if you are playing a decent difficulty level. I don't understand why ppl are so averse to just building a couple of something that has limited, but extremely valuable use. War is about angles, speed, and quickness. Human players often skip walls and rarely build castles so XB's mow through their cities fine. In this respect AI are smarter in that they actually build decently difficult cities that take time to capture. The Deity AI can field 3x your military too and will be constantly producing more units and flanking your XB's with knights and the like if you don't kill their cities quickly. So it's not just about your hammers but about giving them less time to use theirs. If you haven't experienced what I'm saying then maybe you should try Deity or immortal level domination and see how well you do without any early siege. You want trebs for castles, and you don't want to wait for cannons. Cannons are awesome though, when they come, and you should build many to start xp grinding for arties.