A hostile fort will prevent an army from moving from one adjacent province to another adjacent province within that fort's zone of control (so typically the army can only leave that province the way they came in, or march on the fort itself). You can get around this with military access, and you'll see some other weird exceptions to this that you more or less just need to get used to. Occupied forts don't exert zone of control or ever block any movement at all, I don't think.
If two forts are exerting zone of control over the same province, movement will be more restricted, and one of the forts will need to be occupied before that province can normally be accessed. When building forts, having every province be under at least one fort's zone of control is helpful against rebels, and will stop advancing armies until the forts are taken. Having every province be protected by 2 forts will severely restrict enemy movement.
But since forts are very expensive, I would only recommend building a few forts along the borders in strategically important locations, like where a strait can be blocked or a narrow strip of land. They're also nice on your capital, since you get the additional fort level and garrison from the natural capital fort. I would also build them in terrain that's easily attacked (somewhat counterintuitively) so you can lift sieges easily. If a large army sieges a fort in the mountains, you basically have to wait for them to occupy it and leave, and then siege it back yourself, which sucks.
Development is increased by spending monarch points on the province screen if you have Common Sense enabled. It's not very useful though, it's basically just something to do if the alternative is paying an ahead of time malus on a tech, or if you really want an extra building slot for a fort or something. If you're outside of Europe, you may never even consider increasing development, but if you're western tech and not expanding too quickly, you will probably hit your point cap while being ahead of time on tech often.
The buildings show monthly income now instead of yearly now I think, so if you're calculating how quickly they'll pay themselves back, take that into consideration. A temple in a good province is usually like 50 years to pay for itself or something, production buildings usually less so, and extra income from trade buildings is pretty much impossible to calculate. If you're planning on owning every province in a node, marketplaces and the like there are pretty useless except for upstream propagation. I wouldn't say buildings are all worse (not costing MP is very nice), though I miss being able to increase goods produced easily to get that production/trade synergy going in controlled nodes. Barracks are probably the best building to spend money on, as well as a few shipyards.