estaban fortificando
-they/you were fortifying
Imperfecto progresivo para el sujeto ellos/ellas/ustedes del verbo fortificar.

fortificar

Dos semanas después de desembarcar en Inglaterra, los orcos normandos habían dejado de quemar pueblos y ahora estaban fortificando una parte de nuestro amado Sussex.
Two weeks after landing in England, the Norman orcs had pulled back from their village-burning and were now fortifying a peninsula of our beloved Sussex.
Palabra del día
la leña