Fire Emblem: Shadow Dragon/Chapter 10

Princess Minerva "Not all of the League's soldiers took Catria of the Whitewings at her word. Some insisted her plea was a ruse, a trap concocted by Princess Minerva herself Marth doubted that, though; he had seen the woman in Lefcandith Valley, and she did not strike him as a sort of conniving menace his followers feared. As the brunt of Grust's might drew closer, Marth led the League to Castle Deil, deter- mined to attack the citadel and rescue the young Medonian princess, Maria."

Begin
First things first, you will want to make for the fortress. Not only does it contain Maria, the prisoner, it is a good place to hold a position against the reinforcements. The enemy thief will take an item from the chest. Make sure to kill him as fast as possible to regain it.

IT IS IMPERATIVE THAT YOU SAVE MARIA AS SOON AS POSSIBLE. Minerva will not join you until you save her. Really, just go save her then move Marth back to recruit Minerva.

Wolf or some strong units (maybe Shiida if she is high level enough) can take the left side around and kill the enemy fliers.

The main army will spawn in around the time you rescue Maria (if you rushed as fast as possible to her jail cell that is). Minerva will follow them. You don't have to worry about Minerva attacking your troops, just position your troops in this way to defeat the horde of enemies:

oooooooooo H   o     M PP  o     RoE ooo RoEoo

ooooooooEo

EEE

Gordin and Merric should be the ranged units. High defence units like a knight or a cavalier should be the physical. Lena is the healer.

Anyway, just wait until it's like only one unit before Minerva, then move in Marth. Kill the unit in front of you and Minerva will automatically talk to Marth...though you must have rescued Maria first (that is partially the reason why you should hold a position like this, you need to move Marth all the way from the jail to the south entrance)

Use a thief to unlock the door to the north and kill the hero to get a Master Seal, yay! Don't use it yet, let's wait okay? Clear the few guards left near the boss and then, boss time!

Nothing more to do here, seize the gate.