There are two. One is inside the top of the tower to the southwest of the palazzo/church, and the other is on top of a little spire/tower in the middle of the northern wall surrounding the palazzo. I had trouble finding that one too, because there's no roof over it and it's nigh-impossible to see during the day. The blinding white light from the glyph blends in perfectly with the illuminated white tiles in the daylight, so it looks like part of the floor.
There's only one way to get on top of the palazzo itself and that's back through the northeast(more like eastnorth, because it's on the right side of the building). There's a row of scaffolding boards leading up to the roof. But the glyphs aren't located on the palazzo, just very close by. You should be able to climb up the outer wall in any old place. There's a detailed map with pictures here if you get too tired of looking.
There's no way over the fenced sections, those areas are meant to be impenetrable. But trust me, the glyphs aren't even inside the red restricted area. They're just in that little coutyard area in the tall southwest tower and on the outer northern wall.