@camelotkid @Hero1000 did a pretty good job discussing how 2r1d from Rhun can 1r1d Northmen (I play low luck, so odds are ~5% higher), but I disagree with his buy recommendations. Here's my strategic analysis.
Generally Evil starts with a momentum advantage and an IPC disadvantage, and its main goal is to turn that momentum into territory before Good can build a big enough army to stop them.
The main way to do that for Rhun is to quickly take Dale and Rhovanion, but Rhovanion is long and Dale is behind the Iron Hills. If Rhun doesn't hold both regions by turn 3-4, Evil probably loses that front.
With boats, Rhun can take the riverbanks of Dale and Rhovanion as well as the Eastern Iron hills to act as a bridge for their cavalry to conquer a couple turns faster. They can also bring up siege units and infantry to threaten the halls of Gror and the capitols a few turns earlier.
Celduin 2 and Carnen are particularly good for Rhun because a 2-move dromund can move to Dorwinion and ferry units back every turn, saving at least a turn of move.
If Celduin 3 is open, Rhun can also threaten to send troops to the Long Marshes and attack the Elvenkings Halls, but this is usually a lategame worry, when one side has probably already won
The Northmen want to stop this.
The most basic move is to bring your fleet to Celduin 2 and build a raft there. This stops Rhun from being able to destroy your fleet or land in Rhovanion. In addition, if Rhun moves its fleet to the Carnen, you can move in behind them and kill any newly-produced ships or stop crucial resupplies.
If you didn't buy the raft and Rhun moves to Carnen, you'll only have a 77% chance to kill even a lone dromund, and if you lose the raft when doing so, you lose your fleet the next turn.
If you also buy a dromund, you threaten to kill Rhun's fleet the next turn. If also, there's a good chance they'll just give up, because they don't have the IPCs to match you. They'll just ferry everything they can to Carnen and take the East Iron Hills early, then die the next turn. If they don't give up, then they can't move their fleet so it's even better.
If you only buy the dromund, they might try the charge in hope of defeating you in detail. In fact, if they buy a dromund and a raft, they'll be in a good spot even if they get quite unlucky and retreat with both rafts lost and none taken.
You could also do the super aggressive move of buying raft fleets to defend both Celduin 2 and Carnen, which stops them from being able to bypass the Iron hills turn 1. But it's an expensive move (20-24 IPCs) and might cost you the Misty Mountains.
