The Claiming of a Virgin King, by April Andrews MM

When his father is killed in battle, William De Gulliame is thrust into the position of king. But armies of rebel aliens attack from the north, his own people in the south cower below him, and William is in increasing danger of losing his throne. He needs help and he needs it fast…and there is only one male who can give him that help, Lord Dargan of the island fortresses.