Where to use pennants in Castle Siege

With Briton pennants, you can research resource protection as well as longbowmen. I recommend prioritizing longbowmen, since the percentage offered by resource protection is only 5%, and that assumes that you get your military technology building for your civilization all the way to level 6.

In terms of where to spend pennants in general, remember that you only spend pennants to research technologies and that certain technologies require certain pennants. Leveling up technologies that require a certain pennant will also require that same kind of pennant when leveling it up again. The same idea goes for unique units. So, prioritize the upgrade that is better. If you like the technology that is better than requires a certain kind of pennant, go for that. Otherwise, level up the unique unit. If you have pennants left over, then research the other upgrade.