Age of Empires II: Definitive Edition

The aforementioned Old Tower is a fortification on a hill amid a few palisades, just outside and to the northeast of the city that is your main base. The real threat to it will come from the red Teuton forces to the northwest of you: they will field groups of rams to start, and then start incorporating trebuchets, crossing the water at some shallows to the northern corner of the map.

While the red forces are the main threat to you militarily, the yellow Italians to the southeast are the major economic obstacle, as they control two of the trade routes for the badly-needed supplies you get periodically through this mission, and destroying their fortification on the river grant you two transports' worth of free units. (These also have genoese crossbowmen and conditierri, who are great counters to your army, especially your gunpowder-based wagon unique units.) As such, I recommend being aggressive early about destroying enemy barricades and getting some bombard cannons to roll on yellow as soon as possible. A castle on its own will not protect you from red, so make sure you have a detachment of cavalry or similar to deal with siege units. Playing at a slower speed will help you not get surprised by an enemy attack while you're focused on action across the map.

Once yellow is knocked out, it's much easier to wipe out red since you have a lot more resources coming in (with the final enemy, green, never being a threat to you outside of small attacks from the southwest.) Achievement will pop if the tower is still standing on mission completion.