"Hey, remember all those cool, useful features the previous title had? Yeah, let's not add any of those. Actually, let's remove even more things than the predecessor had. That'll make a good sequel."
That's the best guess of what went through the developers' minds when designing Stronghold 3. The only area that has seen an apparent improvement is the graphics. The third installment features fewer units, smaller and less maps, the loss of functions like being able to see inside buildings, and fewer types of buildings as well. It is simply an all-around downgrade except for in the graphical department.
The general consensus seems to be to completely ignore this game and to buy Stronghold 1 or 2 instead.