HELP please with Tarred and Feathered (HoTA)

Ugh...I've just had to restart the map, because at SOME point I was offered 6 sea serpents for doing something but my army was full, so I rejected them. Turns out I needed them to open the gate near the green portal so I can't complete the level and there are no dwellings to get them from (and the town can't build the structure).

Before I get too deep into the level (again!!!), is there any other thing that's a level breaker that I need to be aware of! I was so close to the end, I only had one more fight and it was done!!