This is a fun comparison between games.
On one hand, you have Adventurer Conqueror King System, a modified B/X system that focuses on end-game kingdom building, politics, mass battles, hereditary storytelling, and realm management.
On the other hand, you have Forbidden Lands, a dark survival horror fantasy adventure old-school like game that focuses on exploration, travel, survival, sandbox world generation, exploring adventure sites, building a stronghold, and telling a story of zeroes to heroes.
You probably guessed why these two games are being compared. They both have base-building aspects. Where they differ is in the scope and "why" one builds.
Forbidden Lands Strongholds
Forbidden Lands has this JRPG feeling when it comes to constructing a stronghold. You pick a spot on the map and build, and management consists of building structures, handling the periodic rules for them (like output of resources), and gaining mechanical game benefits from building certain buildings in your home base (like a shrine increasing willpower). Your stronghold can be a part of random events, and you are sometimes pulled into story arcs involving your home base.
Strongholds in this game very much feel like "endgame party resources and upgrades," and in that JRPG sense they fit in well. If the first act of the game is "not dying," then the second act is "stronghold construction," where the final act is confronting the end boss. Again, it feels very much like a traditional JRPG story arc and it is a very fun experience with a beginning, middle, and end.
I do like this sort of three-act structure to a game, and also knowing nothing about the world and moving things along until a story-like end is reached for the group. You could play on the same map again, but the real fun is in not knowing what is out there and building the map for that first time yourself.
But where the construction game ends is in the stronghold. The game says you can build multiple ones, but the extent of the scope of this game is just in building and defending the structures, and constructing upgrades for each one. There is a game-bonus limit to 50 guards in the stronghold as well, so the scope of these are more to guard the stronghold than it is to protect an area of land or several surrounding villages. It is a nice system, very detailed and "upgradey" like a video game, but it is not really a kingdom management style of game.
Okay, completely different. You are conquering and building kingdoms. You are less getting mechanical benefits from building a certain building than you are getting into the mechanics of managing a kingdom that expands, settles, builds, mines resources, clears farms, builds roads, establishes lords and ladies of the land, and deals with its neighbors across the map.
Your scope is much larger in ACKS. Where in Forbidden Lands you could build a road for some reason (make up some rules and get out a sharpie), in ACKS you need to build roads and change the map. Same with terraforming the land, if you need to clear lands for farms, you clear the land, and your adventuring party may get called in to deal with monsters and lairs across that farmland. In ACKS you clear out hexes of mountains to start mines and capture water resources for your cluster of towns downstream, so those orcs don't dam up the river and kill your crops and populations And you raise armies and join in the mass battles with ACKS as well, so you can join in the fun.
You are also not really building smaller buildings with specific bonuses in ACKS inside a castle or keep, such as a workshop or shrine, and expecting some mechanical benefit in play. If you build a shrine in a castle and it makes sense for it to be there, great! That may come up in RP, it may be required for clerics, and the referee is free to use that fact for any reason or purpose - or it may not come up at all, who knows. Maybe it increases loyalty? Also, building mines, fields, and other structures are done by the population you attract instead of you. You for story reasons and increasing your kingdom's resources and wealth may finance a mine with your gold, and that would come up during RP.
ACKS does not really have an end. You conquer and build your first kingdom with your first set of characters, advance the clock 20 years, and start a new party of level one characters while your now older party becomes the regents and rulers of the world, and they can still play but they most manage the realm these days, do political stuff, and make appearances when something important happens. You keep advancing the story, get your characters married and put in positions of power, and generationally continue the story through play. It is very much like a game of Civilization or Crusader Kings that ends when you choose after several generations of rulers.
You can run merchant ventures, run rogue hideouts and do missions for gold, build dungeons as a mage and harvest them for monster parts and treasure, and do all sorts of cool things that are more world-focused than party-focused.
Both Great, Different Reasons
Both are great games, and they both do different things. If you want that lower-level, story-focused, survival, map building, zero to hero experience that is almost like a novel - play Forbidden Lands. This game is also great if you want to play to a conclusion of a story or campaign. Maps are build-as-you-go and filled with the unknown. The game is tightly focused, has a party-based scope, and fun.
If you want a B/X style game that does a whole boatload of stuff at higher levels, more than just fighting bigger monsters than bigger numbers, ACKS is a great game. This is a great game for open-ended play where there is always another land to settle, intrigue between major world powers to be had, or just this map-level management and 4X play that varies across classes and kin. Maps can be established ones or just pure random hex-crawls. The game is broadly focused, grows in scope, and fun.
But know your games and play the one with features that you are looking for. This comparison is not a "one is better" but more of a "know what they do best" sort of contrast. Pick and play from there, and have fun!