That's the western gate of Moria. The Nanduhirion map was made by EA and has simply been changed to work with the expansion plot building system of Edain 4. I think there were no visual changes made (and I don't think they'd be necessary).
Don't know what have been modified, if i remember this map was from BFME1 who already use build plot system, but i don't know about it.
No it's simple some decoration to make remember what is the map, simple details can be really nice to see (same as the hidden stairs in Minas Morgul map, or the 3 trolls in Weathertop ^^)