Building forts definately helps. I left SAlona, Thermon, and Appolonia completely undefended and MAcedon didn't betray me for a long time, I betrayed them. Even one unit in a fort is a deterent because they know that they will have to spent at least two turns sieging the fort before they can move onto the town, by which point you could produce a garrison so they would have to spend at least two more turns sieging tha town, by which point you could have reinforcements their. Not that I would want to send reinforcements back to my center, but the possibility is enough to deter. Thats why stationing armies in the field is usually allot better than garrisoning. They can both reach the city if need be to reinforce at the siege, but can also have a longer range by being posted at range limit away from a city. Ships obviously increase your range dramatically, which is why control of teh sea is so important. Think how far you can move an army in 4 turns by ship. Now thats a deterrent. Also sheer size and power helps. Yes they might pick up a territory, but they also might lose their entire nation.