1) Racial Differences: Avians don't need shipyards and Halflings need less farms than other races for example. Take into account thes differences when building.
2) The Build changes as the game goes on: Early in the game your military may only represent as little as 20% of your population. By the end of the game it may be 300% of your population. There are other changes as well. Just remember than changing builds have an impact.
3) Varying Strategies: There is no one "correct strategy". Anyone who says that there is only one way to succeed or that this or that building is always useless is full of it. Make your own call as to what you should build and go for it.
Farms: Farms feed 175 people after rot. With farms you just need to make sure you build enough for your people to eat. If you figure 1 farm for ever 3 homes or so (or a little less than that) you can make it fine. Use those Fertile Lands, Nature's Blessings, and Rob Granaries to supplement your food if you start running a deficit as large as 6k bushels of food like I did at times last game.
Plazas: I don't build plazas. When they were at 60 people employed each, they were worth it. At 30 each, I don't think they are. Some people like them, but I think relying too much on plazas will come back to burn you near the end of the game as your % of military increases and you have to build huge %'s of homes just to stay even.
Mines: I'm not a big fan of mines anymore. At one point last game I was making 220k a day and my 10% mines was accounting for a whopping 20k of it. Now mines have been reduced further to 30 gc each. Every little bit of cash counts but I would personally consider building mines this round unless I was a Dwarf and could cast Miner's Mystique to increase their production.
Mills: They may replace mines in my lineup this time because they've been improved enough to make them worthwhile for me. Will they be of use for most players? Debatable. I had 8400 acres and at the end my buildings cost 21,345 GC each. If you really don't plan on getting huge next round, they probably aren't worth it to you.
Armouries: Armouries are a necessity. No need to even discuss it. The only people who may not need armouries are people who or whatever reason don't see building a lot of troops. So basically, land farms don't need armouries. Are you a land farm? No? Build armouries.
Barracks: In my personal opinion barracks are only worth it if the race you are playing has an attack time bonus and you want to try to get in more than one full set of attacks each day. If I were going to play an Avian or an Orc next round I would build 10% barracks. If you are a real fanatic you could use it for extra dwarven attacks too but their small bonus means it's probably not worth it to me.
Strongholds: Strongholds would've come in handy about 50 times during the game for me. With people building more and more defense a 30% bonus to attacks would be incredibly helpful. However, I generally go on the theory that's it better to build 30% more troops and get the NW rather than just get a 30% bonus.
Guardstations: I'm toying with the idea of building guardstations for the first 1k acres when I'll be most vulnerable defensively and then tearing them down. For Orcs & Avians these may even save you space building your crappy defensive specialists so you can concentrate on elites. With Dwarves and Halflings this may give you more confidence when you send out those elites. But once again, should you build guard stations or just build 30% more defense? As a top 50 Orc I had guard stations. As a top 50 faerie with 90k Beasties I didn't. Your call.
Banks: I started the game with 27% banks last game and finished with around 9% because the rest went into homes. Banks have been improved now making them even more useful. Some people will tell you banks stink and can never beat a plaza/homes set up. Don't believe it. If you make enough banks they can be very valuable to you.
Hospitals: These are a little frustrating for me to deal with. Originally we were losing 13-17% of our attacking army on "successful" attacks. Then I changed my build in response to that. Then they lowered the rate to 6-8% making the hospitals much less useful. then near the game, losses went way back up again. It was most annoying. If you're an Orc or Avian attacking twice a day I'd seriously consider hospitals. For the other races I'm not as high on them. On the other hand, who the Hell knows how much the combat losses will fluctuate?
Guilds: Spells have become more important this round than last round. So have guilds. Your self spells will work based on what % of guilds you have. Don't skimp on the guilds people. Too many people build 2 or 3 % guildsand then get in a fight and get torn up by someone's wizards. Don't be that person. At the end of the game this time I finished with 1 wizard per 2.5 acres. My kingdom got in a war and between that and magic shield, maybe 1 in 5 spells was working on me. Eventually they pretty much gave up trying to cast on me. Guilds will be just as handy for you.
Towers: Personally, I find that 5% towers cuts it for me for the most part during the game. I'm toying with the idea of building more this time around but I will probably stick with the tried and true 5%.
Watchtowers: Originally I thought these would work like a clear sight spell in which case they'd be a necessity. However since they really just give you a 30% bonus they aren't as valuable. Even with the 30% bonus you can't protect yourself from the dinky little thief province with 200 acres and 3k thieves. This could be of use but in the savage race to save space, I probably won't build them. It doesn't mean they aren't useful however and you may want to consider them if you kept getting ripped up by thieves this round.
Schools: Another interesting building. Let's say you have 20% alchemy science and you have 10% schools. You add another 6% to your income and 6% to your population which may translate into 6% more direct income plus you get all the other benifits. Hell, how can that not appeal to you? The hitch is how many points you can put into science. Early last round I had no money for the sciences making this option less than useful. At the end it would of been incredibly useful when my sciences were buffed up. So do you build it early and see what happens, tear down other buildings later in the game and replace them with schools? Hard to call. But if you invest heavily in sciences this is probably worth some of your valuable space.
Universites: I spent 10% of my space to start the game on universites last game and then promptly put almost no money into science until I hit around 1300 acres after I'd torn them all down. This game my citizens wil remain illiterate because I won't touch universities with a 10 foot poll. Does that mean you shouldn't either? No, it doesn't. If you see yourself spedning extravagant amounts of cash on sciences next game and you didn't max out your sciences this game, they might be worth considering.
Shipyards: Last game I built 250 shipyards because I was part of a massive war against island 1. This round I built 100 total (50 after my kingdom got into a war). Even though I had 50k swordsmen, those 100 shipyards would've been totally adequate to carry all my troops under the new rules. For fast growing provinces I think 2 shipyards per 100 acres is probably plenty. If you are growing slow and really making hordes of troops you could possibly need 3 per hundred acres but I doubt it.
Last but not least, if you are new to the game and need more in depth explanation of what each building does and what the exact %'s are for each building, read the Utopian Guide. If will give you the necessary details. Don't forget, 10 minutes of planning now can make your province end up 2 or 3 times bigger next round. Make a blueprint of what you will build per 100 acres. It's worth it.