In FE8U, it is an event of 0x9FF354.
In FE8J, it is an event of 0xA7140C.
I also write it in the FAQ,Castle Frelia is MAPID: 0x38.
FAQ:Where are Mulder and Vanessa?
Mulder and Vanessa are in Frelia Castle.
Map ID 0x38 It is Frelia castle.
Their unit arrangement is 0x8B43D0 (FE8U)
In FE8J, it becomes 0x908958 (FE8J).
With FEBuilderGBA, looking at MAPID: 0x38, you can see a map with only one tile in the plain.
The event you are looking for is the start event in this chapter.
Open the start event of MAPID:0x38 and scroll down ...
5000 Gold There are events to acquire.
This chapter has a special structure, it will start again when clearing ch 8 and returning to Castle Frelia.
At that time, the subroutine is executed by the flag branch at the beginning.
In this, the event of the invasion route branch is listed.