The Wall is part of your base's defense. It requires Base level 6 for construction and can be developed through 30 building levels.
How defending squads matter
The Wall and the heroes defending it are separate parts of a defense. Red Frenzy explicitly requires allied squads to defend walls; failing to assign them can result in a lost defense.
Garrisoning another commander's base uses a squad assignment. The game prevents sending another garrison squad to the same commander when you already have one deployed there. That occupied squad is part of the marches you are managing, rather than an extra army created by the Wall.
Wall levels and other defenses
Wall building power rises to 316,600 at level 30. That value is the building's power, not the power of the heroes and units assigned to defend.
The Alert Tower affects scouting information. Improving it does not select your Wall's defenders. Hero development and the Infantry, Tank or Aircraft Center affect the relevant heroes, while Wall construction remains its own upgrade path.
