Mortar at Town Hall 5: max level 3. At Town Hall 18 the cap is level 18. Max level per Town Hall, with how many you can build in brackets: TH3 1, TH4 2, TH5 3, TH6 4 (2), TH7 5 (3), TH8 6 (4), TH9 7 (4), TH10 8 (4), TH11 10 (4), TH12 12 (4), TH13 13 (4), TH14 14 (4), TH15 15 (4), TH16 16 (4), TH17 17 (4), TH18 18 (4).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.