Jamieson, Fausset and Brown on Zechariah 9:12

12. stronghold--in contrast to the "pit" (Zec 9:11); literally, "a place cut off from access." Maurer thinks, "a height" (Psa 18:33). An image for the security which the returning Jews shall have in Messiah (Zec 9:8) encamped about His people (Psa 46:1, 5; compare Is 49:9; Pr 18:10).

prisoners of hope--that is, who in spite of afflictions (Job 13:15; Psa 42:5, 11) maintain hope in the covenant-keeping God; in contrast to unbelievers, who say, "There is no hope" (Jr 2:25; 18:12). Especially those Jews who believe God's word to Israel (Jr 31:17), "there is hope in the end, that thy children shall come again to their own border," and do not say, as in Eze 37:11, "Our hope is lost." Primarily, the Jews of Zechariah's time are encouraged not to be dispirited in building by their trials; secondarily, the Jews before the coming restoration are encouraged to look to Messiah for deliverance from their last oppressors.

even to-day--when your circumstances seem so unpromising; in contrast with the "day of the Lord," when Zion's King shall come to her deliverance (Zec 9:9).

I will render double--Great as has been thy adversity, thy prosperity shall be doubly greater (Is 61:7).