I currently have Boromir, Gandalf, Officers, and the WoMT and KoMT boxes. What should I be looking at next?

Denethor for a cheap character, and Fontain Guard for the Fight value?

I'm not sure what Citadel Guard are for, as they seem to fall into the same spot as Fountain Guard.

Citadel Guard and Fountain court are good options to get some higher fight value into the army. If you're running Boromir with the banner you can of course get the normal troops up to fight 4, or get the elite troops up to 5, plus a big re-roll effect.

A pack of plastic rangers could be a good choice to boost the numbers. The rangers with spears and bows have better shooting than normal troops and can spear support behind their armoured colleagues to lend their fight 4, making them a good second rank.

Faramir could be a nice additional hero, while Aragorn of the Black Gate would give you the option to run the army either under Boromir, Gandalf or Aragorn.

The Denethor and Irolas pack and a set of fountain guard is probably a good investment in terms of building your collection. The fountain guard are the top line elites for Gondor and Denethor is a solid way to bring in more warriors. Irolas is just alright; decent for his cost and not a bad model. I think if you are going to do that'd you'd also benefit for getting another box of warriors as well so you have enough to take advantage of the lower cost heroes you are bringing.

As for citadel guard, their claim to fame is really the combination of longbows, bodyguard and horses (not that the horsed models exist). They are good for sitting on rear objectives and shooting at models that try to skirt around and capture. As spear supporters or dedicated archers, they are outclassed by the fountain court and rangers respectively. I've got six of each, but they don't see much use unless I use them as described above, I'm doing a guard of the white tower themed list or I'm doing a scenario.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

I'm ordering the various rules and army books next week, so I will toss in a pack of the Fountain Guard.

I have also added Denethor to the Forge World shopping list.

Can only agree with what has been so said so far.

I do not play this faction myself - just having played against them and read/listend up to a lot of discussions - so take the following with a grain of salt

Osgiliath Veterans are in general shunned because for a few points more, you get warriors (Citadel Guard, Fountain Guard) with better profiles and rule. So unless you plan on a thematic Osgiliath list, you can leave them out.

Don't know what your opinion on warmaschines is but I definitely would go for an Avenger Bolt-Thrower - it sounds like a lot of fun and gives you area control

Knights of Minas Tirith are a good cavalry option and can provide you with speed.

Herowise it depends on where you wanna go - with the Denethor pack you have a good start. You can add more smaller heroes and most of them are good.

For more points most people go either for Boromir or Gandalf the White or Aragorn King Elessar.

Rangers as support with spears and bows behind your warriors are also good.

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