The new ALeP cards do not even nearly have enough reviews as they deserve, so here I am, writing one. In my opinion, their allies are one of the best designs not only in this expansion, but even in the view of the last cycles, and the Rammas Sentry is no exception. But what does make this card so great?
First off, it does not limit deckbuilding choices. While it does seem like a slightly different version of Honour Guard, it works different enough to not be either better or worse than the guard, so that you are not limited in your choices of deckbuilding by having to prefer one over the other if you want your deck to be as good as possible. Firstly, Rammas Sentry only cancels damage on Gondor characters, while the Honour Guard cancels damge dealt to any character. Second, while Honour Guard has to exhaust for his ability, Rammas Sentry doesn’t. You just need a resource in every hero’s resource pool. Honour Guard has a handy Valour Response and a good pool of hitpoints, while Rammas Sentry offers you a nice attack support with his 2 (or 3 with Boromir). Thus, these two cards, while appearing to have the same basic ability, actually work different enough and have different strengths and weaknesses to justify to even put both in your deck (for maximum damage cancellation).
Second, the Rammas Sentry is devoted, which means you can play him even outside a tactics deck if each of your heroes has the Gondor and/or Warrior trait, so he can be actually used in a mono-leadership Gondor deck, which is pretty awesome. Of course, having only warriors won’t really help, since you can only cancel damage dealt to Gondor characters, but you can mess around with the equally well-designed contract The Last Alliance released in the same expansion. If you want to go with him in a mono-leadership deck, his requirements are actually quite easy to meet. The "Wealth of Gondor" archetype, where you want to equip your heroes with as much resources as possible, has been around for quite a while, and Children of Eorl is expanding this archetype with their new allies. And Rammas Sentry fits right into this type of deck, while also not completely excluding other approaches to a Gondor deck. Great design.
Third, the ability. Enough has already been said in the reviews of Honour Guard about the power of damage cancellation, so I will make it quick. It is powerful. Very powerful. And since you need not exhaust the Sentry for his ability and it is only limit once per phase, you can actually e.g. cancel a damage on a questing character in the quest phase and additionally, in the combat phase, rescue your Defender of Rammas from the infamous "deal one damage to the defeding character" shadow effect; and after that he can even help you kill your enemies with his 2 ; and all of this for the cost of 2 resources.
This ally is just pure and utter perfection; congratulations to the designers of ALeP for adding this character to the ever-growing collection of great tactics allies!