Tomen yr Allt
| Location | Near Llanfyllin |
|---|---|
| Region | Powys, Wales |
| Coordinates | 52°46′49.33″N 3°17′45.71″W / 52.7803694°N 3.2960306°W |
| Type | Motte and Bailey Castle |
| Part of | Kingdom of Powys until 1160, then Principality of Powys Wenwynwyn. |
| Length | 40 m |
| Width | 40 m |
| History | |
| Material | Earthwork and timber |
| Founded | Possibly 10th century |
| Abandoned | 1257 (keep destroyed) |
| Periods | Medieval |
| Satellite of | Cantref of Mechain |
| Associated with | Lords of commote of Mechain Uwch Coed |
| Events | Put under siege by Llywelyn ap Gruffudd in 1257 |
| Site notes | |
| Excavation dates | 1978 |
| Archaeologists | Clwyd-Powys Archaeological Trust |
| Condition | Ruin - only earthworks remaining and very overgrown |
| Designation | Cadw scheduled monument |
Tomen yr Allt (or Domen yr Allt) was a Medieval motte and bailey defensive castle near Llanfyllin in Powys, Wales. "Tomen ar hallt" is modern Welsh for "mound on the wooded hillside."