Manned guns in the towers at either end of the bridge? Well, of course! Would they be able to hit the other side of the river? Well, of course they would! To me its a simple question, if it could be used that way in reality, then it should be that way in the game... I guess players would have to pay attention then, wouldn't they? In the military, what is your first general order? Answer, "I will quit my post, only when properly relieved!!!" In plain english, that means that you have to pay attention, or the enemy will take advantage of your ineptitude.... Stand your post troop!!! or get stomped on!!! Its the way of warfare!!!
Seriously, these bridges would serve to focus the fighting in the game, vital targets always do!!!!
Indestructable bridges? OMG, your kidding right? Take a great idea, and ruin it!!! Honestly, if ya don't want them to cross? And ya don't have the forces to keep them out? DEMO THE BRIDGE!!!! Make them rebuild it,
or wait the 45mins till it pops!!! Hell, the defensive "Hero Play" would be to dump the bridge into the drink, while its crowded with badguys, LOL!!!