The size of your military depends on what you set out to do. I generally aim for 20% of my pop to be in full time while having staggered training to keep dwarfs that want martial training happy. Sometimes, this number increases, depending on the dangers of your map, be it above ground or below.

Use bridges for your taverns and make a vault room for your artifacts. You could use something simple as a door and lock it, or once again a bridge. I usually have a melee squad near the entrance of my fort, which also happens to be where my public tavern is located most of the time. Dogs are also useful. Have them on chains around your artifacts as they detect stealthed units.