Age of Empires II: Definitive Edition (PC) - Achievement - An Army Marches on Its Stomach
Complete all secondary objectives and destroy all of Dagnajan's smaller bases before Dagnajan and his army march in the third Yodit mission "A Fallen Crown".
The first thing to note about this mission is that there is a (not particularly challenging) time constraint, so try and get the achievement with time to spare. It should also be noted that you won't really be attacked much - Only by Begemder (Teal) and not with anything close to a large force.
We'll be using this initial freedom to build up our economy and advance to the Imperial Age as soon as possible. The general aim will be to get a large group of Cavaliers to roam the map completing all the tasks needed for the achievement.
Once you have a decent force, head South and destroy Alodian Rebels (Purple) and then head further South to meet Alodian Traders who would otherwise give a quest to destroy the rebels had you not already destroyed them, so in return they will immediately become your allies with whom you can trade with.
Following that, head South-East following the coast to reach some enemy docks which you will be asked to destroy also - Proceed to do so. Also around this time, it would be wise to research some anti-Monk technology as next we'll be heading to the top-middle of the map to destroy Orthodox Monks (Grey).
The final side mission to complete before Dagnajans army begins to march is to the South-East of the map. You will need to destroy a Castle, but it's not really protected by walls or units so should be relatively easy pickings.
Once all the side missions have been completed, you need to also destroy Dagnajans smaller bases which are on either sides of the river down the middle of the map (the areas protected by Wooden Walls). If you reach Stone Walls, you've gone too far South and have reached Dagnajans main base, which you should leave alone for now.
Once you've done all the side missions and cleared out Red's smaller bases, the achievement should trigger and provided you haven't been lollygagging, you should still have a fair bit of time to spare.