this is manning the city walls: http://www.cardgamedb.com/forums/uploads/got/manning-the-city-walls-cd.jpg

when I play manning the city walls, do I have to play an army or can I choose not to play it? if I have to, how would my opponent know that I do have an eligible army without looking at my hand?

Yes - if you have an army, you're obliged to put it in play. It's a hidden state, so your opponent won't know. He can ask a judge to double-check… but in most (all?) cases your opponent will just assume your playing by the rules.

It's the same for At the Gates with no maesters in the deck, etc.