1. Watchtowers - whenever you are weary about your neighbours and would like to see what they're up to. Never hurts to have some, kills FOW. In RTW watchtowers seemed to be first targets for rebel troops, which camped immediately near and turned FOW on. A good way to know where to find some training ground for your troops.

2. Forts - I usually use them as decoy, harrassing the enemy with them in their territory, buying me some extra-time to get ready for a bigger move or just blocking narrow passages. In RTW they were never a must if you wanted to dominate, dunno yet about MTW2, still to early, but I don't feel it's such a big change.

And welcome to TW!