Add the following additional effect to the watchtower Infrastructure:

Full movement point units in garrisons can always join ambush battles.

Add the following additional effect to the Fort Infrastructure:

Full movement point units in garrison and city tiles can always join ambush battles.

Add the following additional effect to the Command Compound Infrastructure:

Full movement point units in the city's border can always join ambush battles.

I suggest this to give this underused infrastructure line more utility, while also giving a option, to counter ambush cheese strategys that block your citys on defense