X-Bow at Town Hall 18: 4 of them, max level 13. It unlocks at Town Hall 9. Max level per Town Hall, with how many you can build in brackets: TH9 3 (2), TH10 4 (3), TH11 5 (4), TH12 6 (4), TH13 8 (4), TH14 9 (4), TH15 10 (4), TH16 11 (4), TH17 12 (4), TH18 13 (4).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.