Look at the screenshot. My army from above. Ruin (I need it on the quest) to be near the water, the only way to it is closed to the city centre. I can't enter in city center. Can only attack the city, but I don't want to attack.

Remove the ban on the movement in foreign centers of the city, or give the opportunity to explore the ruins of ground - from the sea!

Quirks of world generation. Sometimes this is bound to happen... and unfortunately, putting in place the specifics constraints required to ensure this doesn't happen means constraints would be very, very heavy.

You need to get an ice shelf during winter by praying for it if it pops up in the Altar of Auriga (requires Shifters expansion).  With an Ice Shelf, you can explore it during winter.  Though it would be nice to be able to explore coastal ruins from a naval vessel since naval warfare is now an option, preventing people from abusing water immunity to grab all the coastal ruins.