Okay, first I'll say I LOVE ancient/fantasy cartography. Second I'll say I love where this is going. Your landmarks are very stylistic but blend with the world very well. Really, keep it up.
What could benefit this piece is some inspiration from real ancient maps.
http://en.wikipedia.org/wiki/History_of_cartographyMany old maps drew cardinal directions at various points of the map(sorry, I don't know the terminology), filling the sea with a lot of overlapping lines. It was important for navigation, but in your case can really fill up a lot of the empty sea, and make the piece feel more ancient. I also like a lot of fantasy maps where they artistically depict shorelines. Either by adding contour lines to the shore or by adding some sort of linework. For example, see the map of Middle Earth:
http://bquick.info/wp-content/uploads/2010/11/middle-earth-map.jpg And this one uses horizontal lines:
http://lighthousepatriotjournal.files.wordpress.com/2008/02/map-middleearth.jpgLastly, things like sea monsters are such are a must in my opinion. What they didn't know, they scribed as dangerous. This adds a lot of interest to maps.
Either way, keep going, please! Maybe I'll do an edit just because I want to play with this.