Proposed features/Defensive structures

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Defensive structures
Status: Draft (under way)
Proposed by: 501ghost
Tagging: defensive_structure=*
Applies to: node Nodes, way Ways, area Areas
Definition: Military constructions or buildings designed for the defence of territories in warfare

Drafted on: 2021-07-02
RFC start: 2021-07-06
Scheme of a (non-existent) fortress according to the bastioned system with the most important parts and preliminary works depicted. Details can be found here.


This proposal introduces defensive_structure=* to tag defensive structures at all modern and historic/pre-modern  fortifications. It is intended for all structures that were built for the defence of territories in warfare. This key replaces several other tags (see #Tagging) and defines tags for defensive structures with an emphasis on user-defined values based on technical terms in fortification engineering.

The proposal introduces historic=defensive_structure to indicate that a defensive structure has historic significance. As a parallel tag, military=defensive_structure is introduced to indicate that a defensive structure is in use by a military organisation.

It is proposed to no longer use military=* for objects that are not or no longer used by a military organisation and are unlikely to see military use in the future. disused:military=* (see Lifecycle prefix) (or military=* + disused=yes) may be used for structures that formerly saw military usage. For defensive structures, if the usage status of the structure is unknown, then it may simply be tagged with defensive_structure=* and/or building=bunker.

The key outwork=* is introduced as an additional tag to specify that a defensive structure functions as an  outwork or  advanced work of a fortification. Notable and common types of outworks are  lunettes and  ravelins.

It is proposed to deprecate site_type=fortification and fortification_type=* to populate similar and overlapping keys in combination with the tag ruins=yes.


Although fortifications are a common sight in many parts of the world, many types of defensive structures still lack approved or de facto tagging schemes. Some structures, especially historic ones, are mistagged as other historic=* objects or not tagged at all. Other structures are tagged as military=* features even though military organisations are no longer involved with them.

In external discussions, support was voiced for a change in definition of military=* to exclude objects that are not in use by any military organisation. This has been included in the proposal.

Regarding the discontinuation of bunker_type=*, the nice thing about the military is that form always follows function, so using a single functional tag for a structure (submarine pen, casemate, hardened aircraft hangar) is nearly always sufficient to describe what the structure is as well as how it is used. This means that all bunker_type=* tags can be safely included in defensive_structure=* with the benefits that they may be further specified with technical terms and grouped in a single key together with other defensive structures.

The tag combination of historic=archaeological_site + site_type=fortification + fortification_type=* would overlap strongly with historic=defensive_structure and defensive_structure=*. Archaeological practices may be temporary, but the structures themselves are permanent. Also, the proposed tags are more inclusive in definition than the existing tags and can be combined with ruins=yes to indicate the current state of these features. Therefore, it is proposed to deprecate the existing tag combination.

Judging by Taginfo statistics, fortification_type=* is mainly used to tag ring forts and hill forts. These values can be included in castle_type=* with the addition of ruins=yes. Most of the other existing values overlap with the proposed key defensive_structure=* and will therefore be updated to historic=defensive_structure + defensive_structure=* + ruins=yes.

How to map

Individual defensive structures should be mapped as node Nodes in the centre of the structure, as way Ways along a centre line or as area Areas along the outlines. The best fitting method depends on the size and shape of the structure. They should be tagged with defensive_structure=* and may receive additional tags.

If a structure is located outside the principal fortification limits, add the tag outwork=* to the structure.

Structures that are no longer verifiable should only be mapped on OpenHistoricalMap and not on OpenStreetMap.


Newly proposed tags

Defensive structures

The following tags for defensive structures are proposed:

Key Value Description Statistics (Taginfo) Sample photo

Defensive structures

defensive_structure bastion A  bastion Komárno - Castle.jpg
defensive_structure caponier A  caponier Gorazda fortress ditch and caponier.jpg
defensive_structure cavalier A  cavalier Malta - Birgu - Ix-Xatt tal-Birgu - Fort Saint Angelo (MSTHC) 02 ies.jpg
defensive_structure counterscarp A  counterscarp Antwerpen Schansstraat fort VIII, reduit met droge gracht en contrescarp (links) - 391094 - onroerenderfgoed.jpg
defensive_structure hornwork A  hornwork Nicpmi-01514-1-birgu fortifications hornwork of the post of castile.jpg
defensive_structure motte A  motte Launceston Castle - - 22242.jpg
defensive_structure ravelin A  ravelin PL Ravelin August-Wilhelm 2015.JPG
defensive_structure reduit A  reduit Reduit.JPG`
defensive_structure roundel A  roundel Fortezza di Sarzana.jpg


The tag outwork=* is proposed as an additional tag to specify whether the defensive structure is an outwork or advanced outwork of a fortification. Proposed values are:

Other proposed tags

These tags may be used together in cases where a military organisation still uses a structure with historic significance.

Bunkers should be tagged with building=bunker as per usual. The tag defensive_structure=* may be added to specify the type of bunker, such as a  casemate or a  pillbox. For very specific types of bunkers and other structures, consider using model=* as an additional tag.

Unchanged tags

Several structures that may or have played a role in the defence of a location already have documented tagging schemes. The following tags may be combined with defensive_structure=* in relevant cases, but they are otherwise not changed by this proposal:

Changed / deprecated tags

The following tags are deprecated (in certain cases) by defensive_structure=*:


Some examples of defensive structures that would benefit from a dedicated tagging scheme:

  • Bastion "De Nieuwe Agtkant", The Netherlands and adjacent bastions used to be tagged as 4 unique historic=fort, while they were only parts of a single larger fortress. They have recently been retagged as not:historic=fort, but a proper tag for bastions would be very helpful.
  • This castle in France has a castle with many objects also tagged as castles. There is only one castle, so that is very incorrect. Like the previous example, a tag for defensive structures would be very helpful to point out differences between individual structures and the castle.

More examples of where defensive_structure=* would be applicable:


Tags to use in combination

  • name=* - if a defensive structure is named. If it has no name, then name=* should not be used and noname=yes may be used instead. Do not use the name of the fortification to tag individual structures.
  • heritage=* - if the structure has a heritage status. Sometimes the entire fortification, rather than any individual structures, has a heritage status. In this case heritage=* should only be tagged on the element that represents the entire fortification.
  • building=bunker - if the structure is a  bunker
  • building:material=* and roof:material=* – primary material used for the outer layer of the structure (e.g. brick). Only applicable if the structure is (tagged as) a building.
  • start_date=* - year of completion
  • abandoned=yes - if the structure has been abandoned and shows signs of decay
  • disused=yes - if the structure is disused
  • ruins=yes - if the structure is in ruins
  • historic:civilization=* and related tags - to describe roughly which civilization has built a historic defensive structure
  • surface=* - for the top surface of (earthen) structures such as glacis.

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