Main goal is Stronghold (SH) level 20 so that you can get Training camp (TC) level 20
Imo focusing on iron storage and mines is perferable to the watchtower, since no one can steal that iron from you. I left my watch tower at level 5 for months before I finally upgrading to level 10 and that was because of the mission reward gems. So iron storage and mines are the next priorities after SH and TC. You’ll need the iron and extra space to keep upgrading those.
Next priorities is farms and food storage. You’ll need the ones you have producing a decent amount of food to keep upgrading mines. Also once you get to Training camp 13 and eventually TC 20– you need a lot of food to do the research and to train at that level (Important: You really do not need to waste precious hams doing the research for every level in the TC. many players have never used certain levels. Top Key Levels are 1, 2, 4, 13, & 20. Other levels some feel are useful enough to be worth researching are 3, 10, 11, & 12. Particularly 10 & 12 if you dont want to wait for training camp 13 to start getting rare heroes)
Next priority houses for recruits. I find if you use one of the three training camps you get early on as storage for troops and food (level 4 or level 11) then you can focus on houses a lot slower than the formerly mentioned buildings. You’ll need room for 70 troops when you get to TC13. So you should be fine leaving your 3 houses at level 7ish or 8ish I think (estimate). When you get to TC 20 you’ll need room for 100 recruits, but SH 20 opens up room for another house so once you upgrade that one to level 8ish or 9ish (estimate) you’ll have all the necessary room. After that you only need to upgrade houses if you want to. Down the way you will find its convenient to have extra recruit space, especially if you want your TC20 going continously along with a couple for training 1 & 2 star heroes to level up your rare, epic and legendary heroes so you’ll probably want to eventually level houses higher. But thats something to worry about in the far future of the game. Talking months down the line.
Finally, Forge and Barracks take last priority. I wouldnt worry about leveling one of your forges past level 7 or maybe 8 in the early months of playing. That selection should give you what new players need. You’ll have 3 forges early on in the game and down the line you get a 4th. It is a good idea to upgrade a second forge to level 5. (You’ll be able to convert one level 5 or higher building into barracks when your SH reaches level 10. And forges are definitely the one building that doesnt help you out massively by having many of them. ) The final 2 forges you can feel free to not upgrade past level 3 or so for a long time.
edit: Barracks, wait to level that. Its good for improving your troops attatched to your heroes but its not worth leveling until much later on. Its not cheap in iron to upgrade and as troops get better it takes a ridiculous amount of hamp to improve them. Ham better used for leveling your heroes and uprgrading your mines. High level troops do make a difference on good heroes but I wouldnt worry about them too much until you have a constant high amount of iron and ham coming in so that it doesn’t take away from your heores and town.
As for the watchtower…ehhh. Its hard to place this one in the list from to highest to lowest priority for freshly new players. The watch tower is definitely a great way to get extra iron and food, once you have a strong enough defense team to protect most of its resources that is. But once you get a firm grip in your arena you’ll probably win more resources than your own WT provides. Also I find leveling it high early on in the game puts a bit of your target on your back since I doubt you’ll want to open up the game every single hour to collect the resources before a good chunk is raided away. Early into gameplay I would most definitely quickly get it to 5 atleast (to make its existence worth it and for the mission reward.) Then after that the next reward is at level 10. You can upgrade to level 10 a bit at slower pace.