I agree with Quaneh and Severitas about the buildings to build and what is not necessary after all researches are accomplished. I wanted to add something I haven't seen anyone else mention here about Inns.

Ever wonder how the top players have a level 250+ attack hero? Besides attacking people and conquering cities all day to get experience, there's another way to get a high level hero: the level 1 Inn.

I like to have 1 or 2 level 1 Inns in my account (not 1 or 2 in each city). You only get 1 hero offered but your Inn can be demolished in like 2 minutes or less and rebuilt back to level 1 with the free speedup, giving you new heroes every few minutes. I've noticed that these heroes are higher in attribute than a level 10 Inn with 10 crappy heroes in it.

What I look for is a level 1 hero with 60 attribute in any of the categories. When I find a level 1 (or even a 2 for that matter) with a 60 in an attribute, I hire them and recheck the Inn for another such hero.

I use that level 1 to attack level 5 npcs over and over WITHOUT upgrading him for many attacks. When his loyalty starts dropping faster, I'll go ahead and upgrade him till all the points are spent. You can get a level 60 or 70 hero with a very high attack, political or intel attribute around 130+ or better very quickly.

IN A MATURE LVL10 ALL-PURPOSE CITY
Barracks: 15
Cottages: 9
Everything else: 1 of each (no stable, inn, forge or workshop)

Sage