If you like Stronghold then you'll love...
I've wondered this for years after spending countless hours on it, I can't face a reinstall, but nothing has the building feature that is so fun to me. Any Ideas?

If your talking about the building of an economy then maybe the settler series. If you are talking about the castle building then I also can't think of anything that comes close to stronghold.

There is a pretty challenging RTS strategy game on GOG that has no building feature, but it has plenty of destruction in the game (I can set a wheat field on fire and watch enemies rush for cover, avoiding the flames at all cost while I hide in a hay bale a few yards away laughing my arse off.). Be warned though, you get only a set amount of troops and if they die you lose. The game is Soldiers: Heroes of WW2. Awesome graphics and gameplay and you can even control a single unit (A tank, an infantry man, etc...)! Other than that, Stronghold 2?

Stronghold is very unique. I can't think of any other game where you can build amazing castles as in this game.
As Ralackk said, if you're looking for economy management, I'd go for the settlers (Settlers 3 being my favourite of the series).
If, however, you want to stick with the castlebuilding, I'd go for Stronghold 2. They have increased the management system (you also have to make sure your population is happy now), and of course, more castle structures to build.
In my opinion, they completely ruined the mulitplayer part, but I believe the game is still worth buying anyway.

I can also recommend the Lords of the Realm games, especially LotR2. It is turn-based while not in combat, and you can't decide in-game how to build your castle (still, you can build them, based on schemes), but you still have to feed your population with various foodstuff (which you get by assigning peasant to barious fields) and produce the weapons for your soldiers. Also, the unit roster is quite similar to that of the Stronghold games.

Try Age of Empires 2.
Apart from that, if you have a DS there is a game called Lock's Quest. It's arguably more of a tower-defense game, but you do spend a lot of time building fortresses and defenses.
If you are looking for something more quirky with a more techie feeling, you might want to try Evil Genius.
Or the all time awesome Dungeon Keeper series.
Theres the very free, very hard, "roguelike" dwarf fortress.
Still , Age of Empires 2: Age of Kings is probably closest I can think of.

Castle Strike is great as well!

I would agree with Dristvaan, Lords of the Realm is a great game if your going for the medievil feel. Its like total war from the 90s.

open beta for Strong Hold Kingdoms!!!